Dermal glands of Xenopus laevis contain a polypeptide with a highly repetitive amino acid sequence

Mature dermal glands of Xenopus laevis contain storage granules with a characteristic ellipsoid shape. These granules contain, as a minor component, a heat-stable, acidic polypeptide with an apparent molecular mass of 75 kDa. Using antibodies against this protein, positive clones were isolated from a cDNA expression library prepared from skin of X. laevis. One of the cloned cDNAs encodes a pre-protein with a typical signal sequence and a mature part of 396 amino acids. The protein contains 33 copies of the sequence Gly-Gly/Glu-(Ala-Pro)2-4-Ala-Glu. Using the single-letter code for the four predominant amino acids, we have termed this polypeptide the APEG protein. Near its carboxy-terminus, one segment has been found with an amino acid sequence similar to that of spasmolytic polypeptide from porcine pancreas and to the human protein pS2.