Binds the poly(A) tail of mRNA. Appears to be an important mediator of the multiple roles of the poly(A) tail in mRNA biogenesis, stability and translation. In the nucleus, interacts with the nuclear cleavage factor IA (CFIA), which is required for both mRNA cleavage and polyadenylation. Is also required for efficient mRNA export to the cytoplasm. Acts in concert with a poly(A)-specific nuclease (PAN) to affect poly(A) tail shortening, which may occur concomitantly with either nucleocytoplasmic mRNA transport or translational initiation. Regulates PAN activity via interaction with the stimulator PAN3 or the inhibitor PBP1. In the cytoplasm, affects both translation and mRNA decay. Stimulates translation by interaction with translation initiation factor eIF4G, a subunit of the cap-binding complex eIF4F, bringing the 5'- and 3'-ends of the mRNA in proximity. The formation of this circular mRNP structure appears to be critical for the synergistic effects of the cap and the poly(A) tail in facilitating translation initiation, recycling of ribosomes, and mRNA stability. Also regulates translation termination by recruiting eukaryotic release factor 3 (eRF3). Interaction with eRF3 is also required for regulation of normal mRNA decay through translation termination-coupled poly(A) shortening, probably mediated by PAN. Loss of PAB1 from the mRNP after deadenylation triggers mRNA degradation. Inhibits the major cytoplasmic mRNA deadenylase CCR4-NOT complex. Is also associated peripherally with COPI vesicles through its interaction with ARF1, and this is required for correct localization of the asymmetrically distributed ASH1 mRNA. Binds to poly(A) mRNA to form a periodic structure with a packing density of one molecule per 25 adenylate residues. Interacts with the nuclear export factor CRM1 and with the importin SXM1. Interacts with RNA15, a component of the cleavage factor IA (CFIA) complex. Interacts with translation initiation factor eIF4G (TIF4631 or TIF4632) and release factor eRF3 (SUP35). Interacts with the PAB-dependent poly(A)-nuclease (PAN) complex regulatory subunit PAN3. Interacts with ARF1, DCP1, PBP1, the Hsp70 chaperone SSA1, and TPA1. Interacts with PAT1 in an RNA-dependent manner. Predominantly cytoplasmic. Rapidly shuttles between the nucleus and the cytoplasm. Can be exported from the nucleus through at least 2 distinct pathways, the main being dependent on the exportin CRM1, and the second requiring MEX67 and ongoing mRNA export. Import is mediated by the importin SXM1. RNA recognition motifs (RRMs) 1 and 2 bind specifically to the poly(A) tail, whereas RRMs 3 and 4 bind non-specifically to polypyrimidine RNAs and may serve to bind to a different part of the messenger or to other RNAs. RRM 2 also mediates interaction with eIF-4G. Present with 198000 molecules/cell in log phase SD medium. Belongs to the polyadenylate-binding protein type-1 family.