Micronemal protein 2 (MIC2) is the key adhesin that supports gliding motility
and host cell invasion by Toxoplasma gondii. With a von Willebrand factor A
(VWA) domain and six thrombospondin repeat domains (TSR1-6) in its ectodomain,
MIC2 connects to the parasite actomyosin system through its cytoplasmic tail.
MIC2-associated protein (M2AP) binds noncovalently to the MIC2 ectodomain. MIC2
and M2AP are stored in micronemes as proforms. We find that the MIC2-M2AP
ectodomain complex is a highly elongated 1:1 monomer with M2AP bound to the TSR6
domain. Crystal structures of N-terminal fragments containing the VWA and TSR1
domains for proMIC2 and MIC2 reveal a closed conformation of the VWA domain and
how it associates with the TSR1 domain. A long, proline-rich, disulfide-bonded
pigtail loop in TSR1 overlaps the VWA domain. Mannose α-C-linked to Trp-276 in
TSR1 has an unusual (1)C4 chair conformation. The MIC2 VWA domain includes a
mobile α5-helix and a 22-residue disordered region containing two disulfide
bonds in place of an α6-helix. A hydrophobic residue in the prodomain binds to
a pocket adjacent to the α7-helix that pistons in opening of the VWA domain to
a putative high-affinity state.
