A novel testis‐specific isoform of the protein phosphatase1 inhibitor I‐2 (Ppp1r2) is expressed in developing germ cells during sperm morphogenesis.

Abstract

The protein Inhibitor‐2 (I‐2, PPP1R2) of the serine/threonine phosphatase PP1 is one of the ancient PP1 binding proteins. An isoform of PP1, PP1γ2, is highly expressed in testis. This isoform, present only in mammals, is essential for spermatogenesis and sperm function. The coding sequence and the protein product of this message were not identified. We now show that this 0.9 kb message is expressed in developing germ cells during differentiation of haploid spermatids into spermatozoa. Using a spermatid cDNA library and RT‐PCR of testis RNA we have shown that the 0.9 kb mRNA arises due to retention of the last intron (intron 4) that is spliced out in the somatic form of I‐2 mRNA. This intron retention leads to the truncated message that generates a protein with a unique C‐terminus not found in the somatic cell form of I‐2. Protein sequence analysis using MS of heat stable testis extracts confirmed the amino acid sequence of the unique germ cell form of I‐2. Remarkably two other protein regulators of sperm PP1γ2, sds22 and I‐3, are also expressed as testis specific isoforms during late stages of spermatogenesis. Studies are underway to determine the significance of high levels of expression of testis specific versions of these PP1 regulators in spermatogenesis and sperm function. (NIH HD38520)