| The innate immune systems of invertebrates are much more complex than previously thought. One example is the highly diverse Sp185/333 gene family expressed in the coelomocytes (immune cells) of the purple sea urchin, Strongylocentrotus purpuratus, after immune challenge. Encoded proteins exhibit significant sequence diversity but share an organizational structure that includes a leader peptide, a glycine-rich N-terminal region, and a histidine-rich C-terminus. The partial positive charges in the histidine patches of the Sp185/333 proteins make them attractive candidates for immune functions that would involve bacterial binding. The putative binding property of the Sp185/333 proteins against bacteria, sheep red blood cells, and insect cells was assessed. Most suites of Sp185/333 proteins from different animals bound to Bacillus, Vibrio, Shewanella, and Idiomarina species, and treatments with low or high pH did not disrupt binding. Binding often occurred against red blood cells, but rarely to insect cells. Studies using recombinant proteins indicated that bacterial binding can occur in the absence of the coelomic fluid (CF) environment, and that the proteins may be able to saturate the surfaces of bacteria. A novel method was developed to isolate natively expressed Sp185/333 from the CF, and these proteins also exhibited bacterial binding properties. The ability to bind to a variety of non-self substrates may be a function of the diversity of the Sp185/333 system, as manifested in the variable amino acid sequences of encoded proteins. These attributes make the Sp185/333 system an intriguing addition to what is understood about the diversification of invertebrate innate recognition strategies in the arms race against pathogens. |
