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Department of Biological Sciences, Rutgers University, Newark, New Jersey 07102
We have investigated the interactions of two
nuclear-encoded preproteins with the chloroplast protein import machinery at three stages in import using a
label-transfer crosslinking approach. During energy-independent binding at the outer envelope membrane, preproteins interact with three known components of
the outer membrane translocon complex, Toc34,
Toc75, and Toc86. Although Toc75 and Toc86 are
known to associate with preproteins during import, a
role for Toc34 in preprotein binding previously had not
been observed. The interaction of Toc34 with preproteins is regulated by the binding, but not hydrolysis of
GTP. These data provide the first evidence for a direct
role for Toc34 in import, and provide insights into the
function of GTP as a regulator of preprotein recognition. Toc75 and Toc86 are the major targets of cross-linking upon insertion of preproteins across the outer envelope membrane, supporting the proposal that both
proteins function in translocation at the outer membrane as well as preprotein recognition. The inner
membrane proteins, Tic(21) and Tic22, and a previously unidentified protein of 14 kD are the major targets of crosslinking during the late stages in import.
These data provide additional support for the roles of
these components during protein translocation across
the inner membrane. Our results suggest a defined sequence of molecular interactions that result in the transport of nuclear-encoded preproteins from the cytoplasm into the stroma of chloroplasts.
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