A Whiff of Tax­on­omy – The Api­com­plexa

by Elio

The phy­lum Api­com­plexa en­com­passes a large num­ber of ob­lig­ate in­tra­cel­lu­lar par­a­sitic pro­tists, in­clud­ing the Plas­mod­ium of malaria, tox­o­plasma, cryp­tosporid­ium, coc­cidia, and many oth­ers. They cause com­mon se­ri­ous dis­eases in hu­mans and an­i­mals, and they thus be­long to the list of im­por­tant patho­gens. Keep in mind that some 300 mil­lion peo­ple world­wide are in­fected an­nu­ally with malaria and that about 1 mil­lion die of it.

Fig­ure 1. A schematic di­a­gram of an Api­complexa. Sour­ce. Fron­tispiece: Mono­cystis, a mem­ber of the phy­lum Api­com­plexa. Source

Api­com­plexa get their name from an in­tri­cate set of struc­tures at one cell end, the api­cal com­plex. This con­sists of a bunch of mi­cro­tubules and se­cre­tory el­e­ments, some with spe­cific names such as the micro­nemes and the rhop­tries. Ad­di­tion­ally, a spe­cial or­ganelle, the api­coplast, con­tains its own DNA and is thought to be de­rived from chloro­plasts.

Api­com­plexa un­dergo a com­plex life cy­cle, in­duc­ing sex­ual and asex­ual stages, usu­ally in more than one host. Think about mos­qui­toes and peo­ple in malaria, or cats and non-fe­lines in tox­o­plasma. Many Api­com­plexa have a spore-like en­vi­ron­men­tally re­sis­tant struc­ture, the cyst.

Suc­cess­ful bind­ing to their de­fin­i­tive host re­quires move­ment of these or­gan­isms in and out of host cells. They use a kind of mo­tion called glid­ing motil­ity, which, un­like ame­boid mo­tion, does not in­volve cell de­for­ma­tion. In the Api­com­plexa the mo­tion is con­veyed by a struc­ture called the glideo­some, which con­sists of a spe­cial kind of myosin called MyoA, plus some aux­il­iary pro­teins that span the space be­tween the two mem­brane sys­tems of these or­gan­isms and push against an actin fil­a­ment. The rate of mo­tion is sev­eral mi­crons per sec­ond.