This family consists of several Plasmodium falciparum SPAM (secreted polymorphic antigen associated with merozoites) proteins. Variation among SPAM alleles is the result of deletions and amino acid substitutions in non-repetitive sequences within and flanking the alanine heptad-repeat domain. Heptad repeats in which the a and d position contain hydrophobic residues generate amphipathic alpha-helices which give rise to helical bundles or coiled-coil structures in proteins. SPAM is an example of a P. falciparum antigen in which a repetitive sequence has features characteristic of a well-defined structural element [1,2].

1: Molecular variation in a novel polymorphic antigen associated with Plasmodium falciparum merozoites. McColl DJ, Silva A, Foley M, Kun JF, Favaloro JM, Thompson JK, Marshall VM, Coppel RL, Kemp DJ, Anders RF; Mol Biochem Parasitol 1994;68:53-67. PMID:7891748

2: Solution structure of a polypeptide containing four heptad repeat units from a merozoite surface antigen of Plasmodium falciparum. Mulhern TD, Howlett GJ, Reid GE, Simpson RJ, McColl DJ, Anders RF, Norton RS; Biochemistry 1995;34:3479-3491. PMID:7893643