MOLECULAR-CLONING OF A PROTEIN SERINE/THREONINE PHOSPHATASE CONTAINING A PUTATIVE REGULATORY TETRATRICOPEPTIDE REPEAT DOMAIN

Two novel protein serine/threonine phosphatases were cloned from a rat fat cell library with probes generated by a polymerase chain reaction-based cloning approach. One of these cDNAs encoded a protein presumably representing the rat homologue of PPV from Drosophila (75% identity of amino acids). The other novel cDNA encoded a protein phosphatase of 499 amino acids and was designated PPT. Its catalytic domain contains motifs typical for protein phosphatases but is only distantly related with PP1, PP2A, and PP2B (38-42% identical amino acids). When expressed in Escherichia coli, the catalytic domain of PPT exhibited protein phosphatase activity (dephosphorylation of phosphorylase a) that was inhibitable by okadaic acid. As a unique feature among other members of this gene family, PPT has an amino terminal extension of 200 amino acids harboring three tandemly arranged tetratricopeptide repeat (TPR) motifs. This domain has previously been found in other proteins involved in the regulation of RNA synthesis or mitosis. mRNA of PPT was predominantly found in brain and, in lower levels, in testis, but was nearly undetectable in spleen, lung, skeletal muscle, kidney, and liver. It is suggested that the TPR domain of PPT may be involved in the regulation of the function of this novel protein phosphatase.