Cyst Germination Proteins of the Potato Pathogen Phytophthora infestans Share Homology with Human Mucins

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Abstract
We have cloned genes of Phytophthora infestans, the causal agent of potato late blight, that are activated shortly before the onset of invasion of the host tissue. The three genes isolated appear to be arranged in a genomic cluster and belong to a small polymorphic gene family. A conspicuous feature of the deduced proteins is an internal octapeptide repeat with the consensus sequence TTYAP TEE. Because of this structural motif, these novel P. infestans proteins were named Car (cyst-germination-specific acidic repeat) proteins. One of the genes, car90, codes for 1,489 amino acids including 120 octapeptide tandem repeats. Car proteins are transiently expressed during germination of cysts and formation of appressoria and are localized at the surface of germlings. The structural motif of tandemly repeated oligopeptides also occurs in a prominent class of proteins, the mucins, from mammals. The P. infestans Car proteins share 51% sequence homology with the tandem repeat region of a subfamily of human mucins. According to the physiological functions ascribed to mucins, we suggest that Car proteins may serve as a mucous cover protecting the germling from desiccation, physical damage, and adverse effects of the plant defense response and may assist in adhesion to the leaf surface.