(S (S (BASENP (PRPD Its) (NN purpose)) (VP (VBZ is) (S (VP (TO to) (VP (VP (VB break) (PRT (RP down))) (NP (BASENP (DT the) (JJ protective) (NN protein) (NN coating)) (PP (IN of) (BASENP (NN HIV))))))))) (PERIOD .))