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Vol. 8, Issue 11, 2267-2280, November 1997




Departments of
*Genetics and
Cyclophilin and FK506 binding protein (FKBP) accelerate
cis-trans peptidyl-prolyl isomerization
and bind to and mediate the effects of the immunosuppressants
cyclosporin A and FK506. The normal cellular functions of these
proteins, however, are unknown. We altered the active sites of FKBP12
and mitochondrial cyclophilin from the yeast Saccharomyces
cerevisiae by introducing mutations previously reported to
inactivate these enzymes. Surprisingly, most of these mutant enzymes
were biologically active in vivo. In accord with previous reports, all
of the mutant enzymes had little or no detectable prolyl isomerase
activity in the standard peptide substrate-chymotrypsin coupled in
vitro assay. However, in a variation of this assay in which the
protease is omitted, the mutant enzymes exhibited substantial levels of
prolyl isomerase activity (5-20% of wild-type), revealing that these
mutations confer sensitivity to protease digestion and that the classic in vitro assay for prolyl isomerase activity may be misleading. In
addition, the mutant enzymes exhibited near wild-type activity with two
protein substrates, dihydrofolate reductase and ribonuclease T1, whose
folding is accelerated by prolyl isomerases. Thus, a number of
cyclophilin and FKBP12 "active-site" mutants previously identified
are largely active but protease sensitive, in accord with our findings
that these mutants display wild-type functions in vivo. One
mitochondrial cyclophilin mutant (R73A), and also the wild-type human
FKBP12 enzyme, catalyze protein folding in vitro but lack biological
activity in vivo in yeast. Our findings provide evidence that both
prolyl isomerase activity and other structural features are linked to
FKBP and cyclophilin in vivo functions and suggest caution in the use
of these active-site mutations to study FKBP and cyclophilin functions.
Pharmacology, and the
Howard Hughes Medical Institute, Duke University Medical
Center, Durham, North Carolina 27710;
Biochemisches
Laboratorium, Universität Bayreuth, D-95440 Bayreuth, Germany;
and
§Department of Biochemistry, Biozentrum, Basel,
Switzerland CH-4056
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