Short communication Plasmodial serine repeat antigen homologues with properties of schizont cysteine proteases 1

Proteases appear to be required for criticalevents in the erythrocytic life cycle of malariaparasites, including the rupture of erythrocytes bymature schizonts and the subsequent invasion oferythrocytes by daughter merozoites [1,2]. Thisconclusion is supported by studies showing thatparasite rupture and invasion of erythrocytes areinhibited by serine and cysteine protease in-hibitors [1] and that the proteolytic processing oflate schizont-stage proteins is required for thecompletion of the erythrocytic cycle [3,4]. A num-ber of schizont protease activities have been iden-tiÞed biochemically [2], but limitations onavailable quantities of protein have made itdifÞcult to deÞnitively characterize these proteasesor to ascertain their speciÞc biological roles.The Plasmodium falciparum serine repeat anti-gen (known as SERA, SERA-1, SERP or P126[5 Ð 7]) is being studied as a potential vaccinecomponent [8]. A number of SERA homologueshave been described, namely serine repeat proteinhomologue (SERPH [9] or SERA-2 [10]) andSERA-3 [10] from P . falciparum and Þve homo-logues from Plasmodium !i!ax [11]. SERA andSERPH have been localized to the para-sitophorous vacuole of mature schizonts [9,12],and SERA fragments are released into the blood-stream near the time of erythrocyte rupture [12].SERA and its homologues all contain a ! 30 kDaOprotease domainO that has similarity in sequenceto papain-family cysteine proteases, particularlynear highly conserved active site residues [13](Fig. 1A). Taken together, available data suggestthat SERA and SERPH may act as late schizont-