rosetteless

Rosetteless (Rtls) is a secreted C-type lectin-like protein of the colonial choanoflagellate Salpingoeca rosetta, one of the closest living relatives of animals. The 119 kDa protein has an N-terminal signal peptide, two C-type lectin-like domains, serine/threonine-rich mucin-like stretches that are predicted to be heavily O-glycosylated, and two internal repeats of unknown function; carbohydrate binding by its lectin domains has not been tested. Rtls is required specifically for rosette development, the clonal process by which a single founding cell divides without separating to form a spherical colony in response to sulfonolipid signals from Algoriphagus bacteria. A mapped splice-site allele, an independent CRISPR-engineered premature-stop allele, and function-blocking antibodies all abolish rosette formation, while cell growth, chain-colony formation and swimming are unaffected. The protein is secreted at the basal pole of each cell; in rosettes it forms a thick extracellular layer that fills the colony interior and connects the basal poles of neighbouring cells, and in single cells and chains it is seen as weak membrane-associated patches and material deposited on the substrate. This localisation, and the fact that the other two genes known to be required for rosette development encode predicted glycosyltransferases needed for Rtls upregulation and secretion, indicate that Rtls acts as a component of the basal extracellular matrix that holds rosette cells together.