Positively charged amino acids placed next to a signal sequence block protein translocation more efficiently in Escherichia coli than in mammalian microsomes.

Positively charged amino acids are known efficiently to block protein secretion in Escherichia coli, when placed within a short distance downstream of a signal sequence. It is not known whether the same applies to protein secretion in eukaryotic cells, though statistical studies of signal sequences of prokaryotic and eukaryotic secretory proteins have suggested that the situation may be different in this case. Here, we show that identical charge mutations in a model protein have different effects on membrane translocation in E. coli and in mammalian microsomes, and that the 'charge block' effect is much more pronounced in the prokaryotic system. This finding has implications not only for our understanding of the mechanisms of protein secretion, but also points to a potential problem in the expression of eukaryotic secretory proteins in bacteria.