RNA-binding protein that plays a central role in myelinization (PubMed:10864952, PubMed:11917126). Also required for visceral endoderm function and blood vessel development (PubMed:11892011, PubMed:16470614). Binds to the 5'-NACUAAY-N(1,20)-UAAY-3' RNA core sequence (PubMed:16041388). Acts by regulating pre-mRNA splicing, mRNA export, mRNA stability and protein translation, as well as cellular processes including apoptosis, cell cycle, glial cell fate and development (PubMed:10535969, PubMed:12467586, PubMed:11297509, PubMed:11917126, PubMed:15568022). Required to protect and promote stability of mRNAs such as MBP and CDKN1B which promotes oligodendrocyte differentiation (PubMed:10535969, PubMed:15568022). Participates in mRNA transport by regulating the nuclear export of MBP mRNA (PubMed:12467586). May also play a role in smooth muscle development (PubMed:14706070). Isoform 1 is involved in regulation of mRNA splicing of MAG pre-mRNA by acting as a negative regulator of MAG exon 12 alternative splicing. Isoform 3 can induce apoptosis, while heterodimerization with other isoforms results in nuclear translocation of isoform 3 and suppression of apoptosis. Isoform 4 acts as a translational repressor for GLI1. Homodimer. Does not require RNA to homodimerize (PubMed:10506177, PubMed:11297509, PubMed:9671495). Able to heterodimerize with BICC1 (PubMed:9315629). Isoform 1 localizes predominantly in the nucleus and at lower levels in cytoplasm. It shuttles between the cytoplasm and the nucleus. Isoform 3 localizes predominantly in the cytoplasm and at much lower levels in nucleus. Isoform 4 localizes both in the cytoplasm and nucleus. Highly expressed in myelin-forming cells. Expressed in oligodendrocytes and astrocytes in the central nervous system as well as Schwann cells in the peripheral nervous system. Also expressed in the yolk sac endoderm, adjacent to the mesodermal site of developing blood islands, where the differentiation of blood and endothelial cells first occurs (at protein level). Expressed in brain, lung, heart and testis. Expressed in neural progenitors of the ventricular zone (vz) during CNS development, but that expression is down-regulated during neuronal differentiation. By contrast, neural progenitors located in specific subdomains of the vz maintain expression as they differentiate and migrate away into the emerging nervous system. These have characteristics consistent with the acquisition of a glial rather than neuronal fate (at protein level) First detected in the neuroepithelium of the head folds at 7.5 dpc. Expression is strongly present ventrally in the nascent brain and neural tube of 8.5 dpc and 9.5 dpc and in the heart of 8.5 dpc. Isoform 1 is expressed in early embryos, while isoform 3 and isoform 4 are found in late development when myelination begins. The KH domain and the Qua2 region are involved in RNA binding. Methylated by PRMT1. Tyrosine phosphorylated at its C-terminus, probably by FYN. Phosphorylation leads to decreased mRNA-binding affinity, affecting transport and/or stabilization of MBP mRNA. The level of Tyr phosphorylation in the developing myelin is highest in the first postnatal week (P7). During the vigorous accumulation of MBP mRNA between P7 and P20, phosphorylation in the developing myelin drastically declines. By the end of the fourth postnatal week (P28), phosphorylation is reduced approximately 90%. Defects in Qki are the cause of quakingviable (qkv). Qkv is a spontaneous mutation resulting in hypomyelinization of the central and peripheral nervous systems. Mutant mice develop normally until postnatal day 10 when they display rapid tremors or 'quaking' that is especially pronounced in hindlimbs and experience convulsive tonic-clonic seizures as they mature (PubMed:8589716). Mice with qkv specifically lack isoform 3 and isoform 4 in myelin-forming cells, while isoform 1 is lacking in oligodendrocytes of severely affected tracts (PubMed:12888522). Mice with qkv also lack the PRKN gene product, suggesting that the absence of PRKN may also affect the phenotype (PubMed:15014970).