RELATIONSHIP OF GENES ENCODING CA2+ CALMODULIN-DEPENDENT PROTEIN-KINASE GR AND CALSPERMIN - A GENE WITHIN A GENE

Ca2+/calmodulin-dependent protein kinase enriched in cerebellar granule cells (CaM kinase Gr) is a neuronal calmodulin-dependent protein kinase whose purification and partial cloning from rat brain has been described. A combination of the polymerase chain reaction and cDNA library screening was used to determine the DNA sequence that encodes most of the remaining polypeptide sequence. The deduced amino acid sequence was confirmed by comparison with the peptide sequence from purified CaM kinase Gr. Analysis of this sequence indicated the presence of potential catalytic, regulatory, and association domains with 42% overall homology to the alpha-subunit of another neuronal Ca2+/calmodulin-dependent protein kinase, CaM kinase II. The degree of homology within the catalytic domain was 58% with conservation of all invariant amino acids. The portion of sequence that extended from the hypothesized calmodulin-binding domain to the carboxyl terminus of the protein was identical at both the amino acid and nucleotide level to the noncatalytic, calmodulin-binding protein calspermin from rat testis. Screening a genomic library with a portion of the cDNA for CaM kinase Gr allowed the isolation of a genomic clone that contained at least 9 kilobases (kb) of the gene for CaM kinase Gr. Analysis of the sequence revealed that the coding sequences for calspermin were contained within the CaM kinase Gr gene and that alternative splicing of internal exons may lead to the formation of the two different proteins, CaM kinase Gr and calspermin.