PABP, RNA recognition motif 2 (RRM2_I_PABPs)
Type: Domain

This entry represents the RNA recognition motif 2 (RRM2) of type I poly(A)-binding proteins (PABPs).

Poly(A)-binding proteins (PABPs) are highly conserved proteins that bind to the poly(A) tail present at the 3' ends of most eukaryotic mRNAs . These highly conserved proteins are found only in eukaryotes; single-celled eukaryotes each have a single PABP, whereas humans have five and Arabidopis has eight. In humans, three lineages of PABP proteins are observed: cytoplasmic PABPs (PABPC1, PABPC3, and iPABP); nuclear PABP (PABPN1); and X-linked PABP (PABPC5) . The mammalian PABPs contain four RNA recognition motifs (RRMs). RRM 1 and 2 are primarily responsible for the high-affinity binding to homopolymeric adenosines, while RRMs 3 and 4 can bind to nonhomopolymeric AU sequences .

Proteins containing this motif include:

Polyadenylate-binding protein 1 (PABP-1 or PABPC1): PABP-1 is the major cytoplasmic PABP isoform in adult mouse somatic cells. It is able to bind simultaneously to the cap-binding complex subunit eIF4G and to the poly(A) tail. Therefore, it has been suggested to play a role in altering the structure and/or function of the translation termination complex. It may have additional functions within the eukaryotic mRNA transcriptome. PABP-1 possesses an A-rich sequence in its 5'-UTR and allows binding of PABP and blockage of translation of its own mRNA .

Polyadenylate-binding protein 3 (PABP-3 or PABPC3): PABP-3 is a testis-specific poly(A)-binding protein specifically expressed in round spermatids. It is mainly found in mammalian and may play an important role in the testis-specific regulation of mRNA homeostasis. PABP-3 shows significant sequence similarity to PABP-1. However, it binds to poly(A) with a lower affinity than PABP-1. Dislike PABP-1, PABP-3 lacks the A-rich sequence in its 5'-UTR .

Polyadenylate-binding protein 4 (PABP-4 or APP-1 or iPABP): PABP-4 is an inducible poly(A)-binding protein (iPABP) that is primarily localised to the cytoplasm. It shows significant sequence similarity to PABP-1 as well. The RNA binding properties of PABP-1 and PABP-4 appear to be identical .

Polyadenylate-binding protein 5 (PABP-5 or PABPC5):PABP-5 is encoded by PABPC5 gene within the X-specific subinterval, and expressed in fetal brain and in a range of adult tissues in mammals, such as ovary and testis. It may play an important role in germ cell development . Moreover, unlike other PABPs, PABP-5 contains only four RRMs, but lacks both the linker region and the CTD.

Polyadenylate-binding protein 1-like (PABP-1-like or PABPC1L): orthologue of PABP-1.

Polyadenylate-binding protein 1-like 2 (PABPC1L2 or RBM32): orthologue of PABP-1.

Polyadenylate-binding protein 4-like (PABP-4-like or PABPC4L): orthologue of PABP-5.

Polyadenylate-binding protein, cytoplasmic and nuclear (PABP or ACBP-67): PABP is a conserved poly(A) binding protein containing poly(A) tails that can be attached to the 3'-ends of mRNAs. The yeast PABP, also known as Pab1, and its homologues may play important roles in the initiation of translation and in mRNA decay . Like vertebrate PABP-1, Pab1 contains four RRMs, a linker region, and a proline-rich CTD as well. The first two RRMs are mainly responsible for specific binding to poly(A). The proline-rich region may be involved in protein-protein interactions. The association of RRM2 of yeast Pab1 with eIF4G is a prerequisite for the poly(A) tail to stimulate the translation of mRNA in vitro .

Polyadenylate-binding protein Pes4 and Mip6.