Molecular patterning of the oikoplastic epithelium of the larvacean tunicate Oikopleura dioica

Appendicularia are protochordates that rely on a complex mucous secretion, the house, to filter food particles from seawater. A monolayer of cells cov ering the trunk of the animal, the oikoplastic epithelium, secretes the hou se. This epithelium contains a fixed number of cells arranged in characteri stic patterns with distinct sizes and nuclear morphologies, Certain house s tructures appear to be spatially related to defined, underlying groups of c ells in the epithelium, We show that the house is composed of at least 20 p olypeptides, a number of which are highly glycosylated, with glycosidase tr eatments resulting in molecular mass shifts exceeding 100 kDa. Nanoelectros pray tandem mass spectrometric microsequencing of house polypeptides was us ed to design oligonucleotides to screen an adult Oikopleura dioica cDNA lib rary, This resulted in the isolation of cDNAs coding for three different pr oteins, oikosin 1, oikosin 2, and oikosin 3, The latter two are novel prote ins unrelated to any known data base entries. Oikosin 1 has 13 repeats of a Cys domain, previously identified as a subunit of repeating sequences in s ome vertebrate mucins, We also find one repeat of this Cys domain in human cartilage intermediate layer protein but find no evidence of this domain in any invertebrate species, including those for which entire genomes have be en sequenced, The three oikosins show distinct and complementary expression patterns restricted to the oikoplastic epithelium, This easily accessible epithelium, with differential gene expression patterns in readily identifia ble groups of cells with distinctive nuclear morphologies, is a highly attr active model system for molecular studies of pattern formation.