Interaction of the virally encoded fungal toxin KP4 with calcium channels
Abstract
KP4 is a fungal toxin secreted by U. maydis, the causative agent of smut disease in corn. UMV, a dsRNA mycovirus, persistently infects a small percentage of U. maydis strains and encodes the ‘killer’ toxins KP1, KP4 and KP6. These toxins are secreted by, but do not affect, the host strain. Therefore, these toxins facilitate a unique symbiotic relationship between the host strain and the virus. Both KP4 and the long scorpion toxin AaHII, a Na+ channel blocker, are members of the α/β sandwich protein family. Because of a small degree of structural homology between these two toxins, it was hypothesized that KP4 might act as an ion channel blocker. This hypothesis is contrary to the prevailing dogma that fungal toxins form channels in the target cell membrane. The work described here demonstrates that KP4 does not form channels, but rather blocks calcium import by reversibly binding calcium channels on the plasma membrane. This process does not kill target cells but rather inhibits cell division. Since cAMP is shown here to abrogate the effects of KP4, budding growth in U. maydis may involve calcium-mediated signal transduction pathways. Patch-clamp experiments demonstrate that KP4 specifically blocks mammalian L-type calcium channels, presumably via interactions with the pore region of the channel. Chemical modification studies suggest that a single lysine residue in KP4 (Lys42) plays a critical role in the interaction of KP4 with the channel. This result is consistent with the initial hypothesis that KP4 and the scorpion toxin are structurally and functionally homologous. Finally, modification of Lys42 abrogates KP4 inhibition of both mammalian calcium channel activity and fungal cell growth. This data suggests that the pore region of the fungal calcium channel targeted by KP4 is structurally similar to the pore region of L-type voltage-gated calcium channels.
Degree
Ph.D.
Advisors
Smith, Purdue University.
Subject Area
Biophysics
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