Gene structure and function of tyrosine kinases in the marine sponge Geodia cydonium: Autapomorphic characters of Metazoa

Porifera (sponges) represent the most ancient, extant metazoan phylum. They existed already prior to the 'Cambrian Explosion'. Based on the analysis o f aa sequences of informative proteins, it is highly likely that all metazo an phyla evolved from only one common ancestor (monophyletic origin). As 'a utapomorphic' proteins which are restricted to Metazoa only, integrin recep tors, receptors with scavenger receptor cysteine-rich repeats, neuronal-lik e receptors and protein-tyrosine kinases (PTKs) have been identified in Por ifera. From the marine sponge Geodia cydonium, a receptor tyrosine kinase ( RTK) has been cloned that comprises the characteristic structural topology known from other metazoan RTKs; an extracellular domain, the transmembrane region, the juxtamembrane region and the TK domain. Only two introns, withi n the coding region of the RTK gene, could be found, which separate the two highly polymorphic immunoglobulin-like domains, found in the extracellular region of the enzyme. The functional role of this sponge RTK could be demo nstrated both in situ (grafting experiments) and in vitro (increase of intr acellular Ca2+ level). Upstream of this RTK gene, two further genes coding for tyrosine kinases (TK) have been identified. Both are intron-free. The d educed aa sequence of the first gene shows no transmembrane segment; from t he second gene - so far - only half of its catalytic domain is known. A phy logenetic analysis with the TK domains from these sequences and a fourth, f rom a novel scavenger RTK (all domains comprise the signature for the TK cl ass II receptors), showed that they are distantly related to the insulin an d insulin-like receptors. The presented findings support the 'introns-late' hypothesis for such genes that encode 'metazoan' proteins. It is proposed that the TKs evolved from protein-serine/threonine kinases through modulari zation and subsequent exon shuffling. After formation of the ancestral TKs, the modules lost the framing introns to protect the evolutionary novelty. Since cell culture systems of sponges are now available, it can be expected that soon also those mechanisms that control the developmental programs wi ll be unravelled. (C) 1999 Elsevier Science B.V. All rights reserved.