Human testis expresses a specific poly(A)-binding protein

In testis mRNA stability and translation initiation are extensively under t he control of poly(A)-binding proteins (PABP). Here we have cloned a new hu man testis-specific PABP (PABP3) of 631 amino acids (70.1 kDa) with 92.5% i dentical residues to the ubiquitous PABP1. A northern blot of multiple huma n tissues hybridised with PABP3- and PABP1-specific oligonucleotide probes revealed two PABP3 mRNAs (2.1 and 2.5 kb) detected only in testis, whereas PABP1 mRNA (3.2 kb) was present in all tested tissues. In human adult testi s, PABP3 mRNA expression was restricted to round spermatids, whereas PABP1 was expressed in these cells as well as in pachytene spermatocytes, PABP3-s pecific antibodies identified a protein of 70 kDa in human testis extracts. This protein binds poly(A) with a slightly lower affinity as compared to P ABP1. The human PABP3 gene is intronless with a transcription start site 61 nt upstream from the initiation codon. A sequence of 256 bp upstream from the transcription start site drives the promoter activity of PABP3 and its tissue-specific expression. The expression of PABP3 might be a way to bypas s PABP1 translational repression and to produce the amount of PABP needed f or active mRNA translation in spermatids.