Expression of a novel isoform of cyclin I in human testis

A new isoform of cyclin I, designated cyclin ITI, was cloned from the human testis cDNA-lambda gt10 library by using a approximately 0.5 kb PCR fragment that was obtained by using primers based on the published cyclin I sequence from the human brain. The cyclin ITI cDNA is 1443-bp long and has an ORF of 178 aa with the first ATG Met start codon at nt 1 and has the well-conserved sequence of cyclin box. The 5' flanking sequence of 728-bp has a high similarity (99.6%) with the 5'-intron of the human beta-polymerase gene. The nt sequence of cyclin ITI cDNA has high similarity (99.3%) with the cyclin I gene sequence. The cyclin ITI was found to have three amino acids mutations at nt 25, 172, 223, respectively, including one in the well-conserved sequence of cyclin box. Northern blot analysis indicated that the cyclin ITI is expressed in human testis at increased level compared to other tissues. The in vitro translated protein of the cyclin ITI cDNA was recognized specifically by the antibodies raised against the human sperm proteins, indicating its presence in human sperm. This new isoform may have a physiological role in spermatogenesis and/or human sperm cell function--especially in capacitation and/or acrosome reaction.