Functional analysis of alternatively spliced tyrosinase gene transcripts.

Three different cDNA clones (pmcTyr1, pmcTyr2 and pmcTyr3) representing mRNAs originating by alternative splicing of the primary transcript of mouse tyrosinase gene, were identified and characterized by sequence analysis and by a functional assay. These cDNAs were subcloned into the newly constructed expression vector pHD. After electroporation of these hybrid clones into tyrosinase negative cells, protein extracts were prepared and tested for tyrosinase enzyme activity. Only the cDNA insert of pmcTyr1 was able to confer tyrosinase enzyme activity. This cDNA encodes a protein 533 amino acid residues in length containing a putative leader peptide of 18 amino acids and six putative glycosylation sites. Comparisons of the deduced amino acid sequence of the cDNA clone pmcTyr1 with the protein sequence of tyrosinases from man, Streptomyces, Neurospora and with haemocyanin subunits from a spider showed two regions of sequence conservation. One of these regions is known to be involved in copper binding. Since this gene with the coding capacity for tyrosinase is absent in all studied c-locus lethal deletion mutant mice, we have evidence that albinism in mice is caused by mutations of the tyrosinase gene.