SIGNAL SEQUENCE PROCESSING IS REQUIRED FOR THE ASSEMBLY OF LAMB TRIMERS IN THE OUTER-MEMBRANE OF ESCHERICHIA-COLI

Proteins destined for either the periplasm or the outer membrane of Es cherichia coli are translocated from the cytoplasm by a common mechani sm. It is generally assumed that outer membrane proteins, such as LamB (maltoporin or lambda receptor), which are rich in beta-structure, co ntain additional targeting information that directs proper membrane in sertion. During transit to the outer membrane, these proteins may pass , in soluble form, through the periplasm or remain membrane associated and reach their final destination via sites of inner membrane-outer m embrane contact (zones of adhesion). We report lamB mutations that slo w signal sequence cleavage, delay release of the protein from the inne r membrane, and interfere with maltoporin biogenesis. This result is m ost easily explained by proposing a soluble, periplasmic LamB assembly intermediate. Additionally, we found that such lamB mutations confer several novel phenotypes consistent with an abortive attempt by the ce ll to target these tethered LamB molecules. These phenotypes may allow isolation of mutants in which the process of outer membrane protein t argeting is altered.