THE CATALYTIC SUBUNIT OF THE CAMP-DEPENDENT PROTEIN-KINASE OF OVINE SPERM FLAGELLA HAS A UNIQUE AMINO-TERMINAL SEQUENCE

The basis for the unusual properties of the catalytic subunit (C) of r am sperm cAMP-dependent protein kinase was investigated. Ram sperm C w as purified and found by mass spectrometry (MS) to be similar to 890 D a smaller than C alpha, the predominant somatic isoform. Partial inter nal amino acid sequence from ram sperm C was an exact match to that of bovine C alpha, but differed from the predicted sequences for the C b eta and C gamma isoforms. MS analysis of 2-nitro-5-thiocyanatobenzoic acid fragments showed that the mass difference originated in the amino -terminal region. A unique blocked amino-terminal fragment was isolate d from sperm C and sequenced by a combination of tandem mass spectrome try and Edman degradation of a subfragment, The results revealed that the amino-terminal myristate and the first 14 amino acids of C alpha a re replaced by an amino-terminal acetate and six different amino acids in sperm C. The predicted mass difference due to these changes is 899 Da, The region of homology between sperm C and C alpha begins at the exon 1/exon 2 boundary in C alpha, suggesting that sperm C results fro m use of an alternate exon 1 in the C alpha gene. The different amino terminus of sperm C may be related to a unique requirement for localiz ation of the ''free'' C subunit within the sperm flagellum.