intracellular signaling pathways transmit signals of various extracellular stimuli to their cytosolic and nuclear targets in order to induce biological responses , such as proliferation , differentiation , cell death and migration .
when needed , the signals are transmitted from the cytoplasm to the nucleus via translocation of one or more components of each of the signaling pathways involved .
thus , after stimulation , a large number of signaling proteins are rapidly translocated to the nucleus to induce and regulate many nuclear processes .
however , despite the importance of stimulated nuclear signaling , the mechanisms by which these components reach the nucleus upon stimulation have been elucidated only for a few signaling pathways .
classic nuclear shuttling is mediated by an importin- complex that binds to cargoes containing a nuclear localization signal ( nls ) , consisting of mono- or bi - partite clusters of basic amino acids [ 1 - 3 ] .
this importin- complex often acts as a housekeeping mechanism that shuttles most nuclear proteins immediately to the nucleus after their translation .
the relocalization of cargoes is followed by the dissociation of the proteins from the importins upon binding to rangtp , which exports the importins back to the cytoplasm , while the cargo remains in the nucleus .
however , only a limited number of signaling proteins , such as nfb   and erk5 ( extracellular signal - regulated kinase 5 ) [ 8 - 10 ] , use this machinery for their stimulated nuclear shuttle . aside from this canonical mechanism ,
importin-   or similar karyopherins , termed -like importins , operate by binding to non - canonical nlss to mediate translocation without the assistance of importin-. the mechanism by which these components operate is much less understood and is currently under investigation . recently
, several -like importins have been implicated in the stimulated nuclear translocation of signaling proteins . here ,
we propose that this group of importins might be responsible for the swift nuclear shuttling of many proteins following various stimuli .
the rapid and robust activation of erk1/2 allows the phosphorylation and modulation of the activity of more than 300 proteins , which are localized either in the cytoplasm or the nucleus [ 13 - 15 ] .
these substrates are important for the induction and regulation of cellular processes , including proliferation , differentiation , and migration amongst others [ 16 - 19 ] .
the sub - cellular localization of erk1/2 plays an important role in its regulation and physiological functioning .
interestingly , it was shown that the nuclear accumulation of erk1/2 is important primarily for the induction of proliferation , while other erk - dependent processes are mostly regulated by cytosolic molecules .
erk1/2 localization , as well as the mechanisms that govern it , has been elucidated over the past decades . in resting cells , all components of the erk1/2 cascade are localized primarily in the cytoplasm due to their interaction with different anchoring proteins [ 25 - 28 ] ( see figure 1 ) . upon stimulation ,
mek1/2 phosphorylates erk1/2 in their tey motif , thereby inducing a conformational change resulting in the activation of erk1/2 and detachment from their anchors .
this detachment exposes erk1/2 to an additional phosphorylation on two ser residues ( an sps motif ) within a nine amino acid sequence , termed nuclear translocation signal ( nts ) .
this phosphorylation can be mediated by both stimulated and constitutively active protein kinases , including protein kinase ck2 and auto - phosphorylation by active erk1/2 .
the phosphorylation of the sps motif allows it to bind importin-7 , which escorts erk1/2 molecules to the nuclear pores , inducing nuclear sliding .
once in the nucleus , rangtp dissociates importin-7 from erk1/2 , and consequently , induces their nuclear accumulation .
it was also shown that erk1/2 may interact directly with the nuclear pores , and it is possible that these direct interactions are able to facilitate the erk1/2 translocation .
in addition , this process may be regulated by calcium , as a reduction in intracellular calcium concentrations was shown to induce faster nuclear shuttling .
the following steps are illustrated : ( a ) binding of erk1/2 with anchor proteins in resting cells ; ( b ) stimulation is followed by phosphorylation of the tey motif of erk1/2 by mek1/2 , and detachment of erk1/2 from their anchors ; ( c ) phosphorylation of erk1/2 on its sps motif by ckii .
( d ) binding of phosphorylated erk1/2 to importin-7 and nuclear sliding through the nups ; ( e ) dissociation of erk1/2 from importin-7 by rangtp , and nuclear accumulation of erk1/2 . for more details , see text .
interestingly , these results in mammals were consistent with findings in drosophila , where dim-7 ( the ortholog of importin-7 ) was identified as the carrier of d - erk to the nucleus .
once in the nucleus , erk plays a critical role in the development of eyes and wings in drosophila .
moreover , while comparing the mechanism of nuclear translocation of components of the erk cascade with other proteins , we established that the nts might act as a specific stimulus - induced and importin-7-dependent nuclear translocation signal for some signaling proteins lacking an nls .
however , since many signaling proteins contain neither nls nor nts , it is possible that other ill - defined -like importins participate in the stimulated translocation , using various non - canonical nlss .
although importin- complexes mediate the nuclear shuttling of a large number of proteins , it is now clear that other karyopherins are required for the translocation of the full repertoire of nuclear proteins .
such karyopherins were initially discovered as nucleoporin - binding proteins , and their homology with importin- suggested a function in nuclear transport [ 39 - 43 ] , which initiated their  importin  terminology   ( see table 1 for nomenclature ) .
subsequently , more dedicated studies identified at least 10 more -like importins in mammals that share a sequence motif related to the ran - binding site of importin- , and can shuttle to the nucleus under various conditions . the -like importins known today share low overall sequence identity ( 10 - 20% ) , and have 19 - 20 helical heat repeats arranged into super - helical or ring - like structure . their molecular weights ( 90 - 150 kda ) , and isoelectric points ( pi = 4.0  5.9 ) vary .
these importins mediate the translocation of proteins into the nucleus under varying conditions , including stimulation . here , we describe the possible involvement of the -like importins , as well as exportin-4 , in the stimulated translocation of signaling proteins .
modified from : eldar and seger mol cell biol ( 2013 ) , submitted . among the -like importins with
the highest number of identified cargoes is importin-7 , which seems to utilize several mechanisms and distinct nlss to shuttle its distinct cargoes ( table 2 ) . in some cases , usually in non - stimulated cells , importin-7 acts in a complex with importin- , or in parallel to importin-2 , importin-4 , and importin-8 .
thus , importin-7 mediates the nuclear translocation of erk1/2 described above , as well as mek1 and smad3 , by binding to the nts sequences of these cargoes .
moreover , it was reported that importin-7 is able to directly bind to a canonical nls sequence in the glucocorticoid receptor to escort it to the nucleus upon hormonal stimulation .
in addition , importin-7 seems to shuttle other signaling proteins or transcription factors to the nucleus in an nts- and canonical nls - independent manner .
such molecules include the transcriptional regulators hif1- , c - jun , several smad proteins , sox-2 , hiv-1 , egr-1 , and the oncogenic npm - alk .
however , it remains unclear whether the importin-7-mediated translocation of all these proteins is affected by stimulation , or is active merely in resting cells .
although the information on other members of the family still lags behind that of importin-7 , it seems that at least some of them play important roles in stimulated translocation as well .
accordingly , importin-8 was shown to induce the nuclear accumulation of ago2   and smad1/4 .
c - jun was shown to be transported by importins 2 , 5 , 7 , 9 , and 13 , that might be related , at least in part , to its stimulated nuclear accumulation .
c - fos to the nucleus after translation   or upon stimulation , and importin-4 escorts vitamin d receptor to the nucleus upon ligand stimulation .
interestingly , exportin-4 , which participates mainly in nuclear export , has been shown to function as an importin for sox-2 , in addition to importin-7 and importin-9 .
this makes exportin-4 a distant relative of the -like importins ( table 1 ) , although it is not clear whether it participates in stimulated translocations as well . in general , -like importins are able to induce both stimulated and/or non - stimulated translocations , using at least 4 mechanisms : ( i ) monomeric , direct canonical nls - dependent cargo binding ; ( ii ) monomeric , direct non - canonical nls - independent cargo binding ; ( iii ) cargo binding in a complex with importin- ; and ( iv ) cooperation with other -like importins ( see more details in table 2 ) .
thus , as a group , -like importins may play an important role in the stimulated translocation of signaling proteins and transcription factors .
there is increasing evidence that the translocation of signaling proteins into the nucleus is much more tightly regulated than it was thought just a few years ago . aside from the nls / importin- machinery , other mechanisms , such as passive diffusion , active transport of homodimers [ 64 - 66 ] , direct binding to nuclear pore machinery [ 31,67 - 70 ] , escort to the nucleus by other nls - containing proteins   and indirect aid by the canonical machinery , were initially proposed for several signaling proteins .
however , some of these findings were not properly verified , and later were either disputed [ 73 - 75 ] , or found to be cell - type specific .
therefore , it is worthwhile to entertain the possibility that at least some of these alleged mechanisms are , in fact , part of the wider -like importin - dependent networks .
in addition , since dysregulation of the signaling proteins described above is involved in diseases , such as cancer and autoimmunity , it would be interesting to study the potential therapeutic implications of inhibiting their nuclear translocation .
several attempts have been made to block canonical nls / importin- mediated nuclear translocation [ 77 - 80 ] .
since many proteins use this machinery to translocate to the nucleus , such inhibition might affect too many processes and may fail to develop into desired specific therapies .
however , a more specific approach might be to target the non - canonical mechanism of translocation , which seems to act within a limited number of distinct proteins upon stimulation . in this direction
this peptide is able to compete with natural substrates and is resistant to ran - mediated release in the nucleus , therefore specifically inhibiting this process .
however , in order to develop strong inhibitors for a specific cargo/-like importin complex , we need to extract precise information on the structural interaction , as well as the regulation of import
. we will then be able to explore this mechanism as a new layer of therapeutic intervention .
 classic nuclear shuttling is mediated by an importin- heterodimer that binds to cargoes containing a nuclear localization signal , and shuttles most nuclear proteins immediately after their translation . aside from this canonical mechanism , kariopheryn-s or -like importins 
 operate by binding to non - canonical nuclear localization signals to mediate translocation without the assistance of importin-. the mechanism by which these components operate is much less understood and is currently under investigation . recently 
 , several -like importins have been implicated in the stimulated nuclear translocation of signaling proteins . here , 
 we propose that this group of importins might be responsible for the swift nuclear shuttling of many proteins following various stimuli . 