Mitochondrial protein import

Transport of nuclear-encoded precursor proteins into mitochondria
includes proteolytic cleavage of aminoterminal targeting sequences
in the mitochondrial matrix. We have isolated the processing
activity from Neurospora crassa. The final preparation (enriched
ca. 10,000-fold over cell extracts) consists of two proteins, the
matrix processing peptidase (MPP, 57 kd) and a processing enhancing
protein (PEP, 52 kd). The two components were isolated as monomers.
PEP is about 15-fold more abundant in mitochondria than MPP. It is
partly associated with the inner membrane, while MPP is soluble in
the matrix. MPP alone has a low processing activity whereas PEP
alone has no apparent activity. Upon recombining both, full
processing activity is restored. Our data indicate that MPP
contains the catalytic site and that PEP has an enhancing function.
The mitochondrial processing enzyme appears to represent a new type
of “signal peptidase,” different from the bacterial leader
peptidase and the signal peptidase of the endoplasmic reticulum.