This is a thrombospondin type I repeat (TSR) found in properdin. Properdin, also known as factor P (fP), is a glycoprotein constructed from a common pool of structure units or modules, which are homologous to the thrombospondin type 1 repeat, TSR. It is positive regulator of the complement system that stabilises the alternative pathway C3-convertase C3bBb. Properdin also inhibits the factor H-mediated cleavage of C3b by factor I. In addition, properdin acts as a pattern recognition molecule capable of identifying and interacting with microbial surfaces, apoptotic cells, and necrotic cells [1]. However this role of pattern recognition is controversial [2]. Studies indicate that this domain is a TSR variants. It is present at the N-terminal of properdin and has been denoted as TSR-0. It is suggested that the TSR-0 domain of properdin which possesses only the six Cys residues and no CWR-layered motif (which is usually found in other TSR domains) may constitute a truncated TSR domain [3].

1: Identification and characterization of properdin in amphioxus: Implications for a functional alternative complement pathway in the basal chordate. Gao Z, Ma Z, Qu B, Jiao D, Zhang S; Fish Shellfish Immunol. 2017;65:1-8. PMID:28366782

2: Properdin binding to complement activating surfaces depends on initial C3b deposition. Harboe M, Johnson C, Nymo S, Ekholt K, Schjalm C, Lindstad JK, Pharo A, Hellerud BC, Nilsson Ekdahl K, Mollnes TE, Nilsson PH; Proc Natl Acad Sci U S A. 2017;114:E534. PMID:28069958

3: The dimeric and trimeric solution structures of the multidomain complement protein properdin by X-ray scattering, analytical ultracentrifugation and constrained modelling. Sun Z, Reid KB, Perkins SJ; J Mol Biol. 2004;343:1327-1343. PMID:15491616