The purpose was to elucidate whether the truncation affected the distribution of the large subunit between the supernatant and the pellet fraction. The surprising result was that the removal of as many as 20 of the 32 amino acids still allowed activity to develop. Truncation of six amino acids more, however, prevented the appearance of activity. The truncated HycE species show the same distribution, with the exception of the form lacking 26 amino acids which is mainly found in the P100 fraction. Whether this reflects loss of solubility or an altered, i.e. tighter association with the membrane remains open.
Truncation also seems to affect the amount of reacting material in the cells.
The most plausible but not only possible explanation is that the C-terminal deletions impair the stability of the mutant forms. Taken together, the results show that processing does not require the intact C-terminal extension. A deletion of 12 amino acids clearly permits processing to take place. Truncated versions lacking 16 or 20 amino acids still develop formate hydrogen-lyase activity, which leads to the conclusion that they must be processed. Removal of 26 residues yields an inactive product.