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residue effects count to be weak. Only in the quintuple mutant, where contacts of ß6 with all neighboring ß-subunits are disturbed, is a notable phenotype seen.

ß7. Based on our observation that the displacement of the mutationally introduced Thr1 with respect to its position in active subunits could explain the failure to activate ß3 and ß6, and based on the perfect match of the polypeptide backbone around Thr1 of ß7 and of the active subunits (Fig. 4g), we then attempted to activate ß7. Two residues have to be replaced, Arg33 and Phe129. The resulting yeast strain was viable and indistinguishable from the wild-type. N-terminal sequencing of ß7 revealed the presence of the wild-type propeptide. In the absence of a crystal structure, we can only suspect that the distortion in the backbone of wild-type ß7 in the region around Phe129, which we attribute mainly to unfavorable interactions with Asp 166 (Fig. 4g), is still present in the mutant and responsible for the inactivity and inability to autolyse. We did not try to activate ß4 because major differences between the Ca-traces of this subunit and of the active subunits exist (Fig. 4h).

Note Added in Proof. While this paper was in press, a publication by Arendt and Hochstrasser (28) appeared suggesting acetylation of ß1, ß2, and ß5 subunits by genetic methods in mutants lacking the respective propeptides. These results are in agreement with our findings in ß1 by analytical methods.

We thank Silvia Körner and Frank Siedler (Max-Planck-Institut für Biochemie, Martinsried, Germany) for help with mass spectrometry, Karlheinz Mann (Max-Planck-Institut für Biochemie, Martinsried, Germany) for help with N-terminal sequence analysis, and G.B. Bourenkow and H.Bartunik (DESY, Hamburg, Germany) for assistance with the x-ray experiments. The Sonderforschungsbereich 469 provided financial support. The work was furthermore supported by a grant from the Deutsche Forschungsgemeinschaft (Bonn) and the Fonds der Chemischen Industrie (Frankfurt).

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