A C-terminal Cys is a rather strange residue to add in protein engineering for expression and stability. Can you provide more context? A literature reference? An unpaired Cys would be a hassle in protein purification and storage, as it might form an intermolecular disulfide bond upon oxidation.

Optimization for translation efficiency is mostly done at or near the N-terminus, to disrupt RNA secondary structures that can interfere with translation initiation.

A single unpaired Cys residue at or close to either terminus may be introduced as an attachment point for subsequent site-specific chemical modification, e.g. fluorescent dyes.

I think I have found the answer.

One form of C-terminal modification is prenylation. During prenylation, a farnesyl- or geranylgeranyl-isoprenoid membrane anchor is added to a cysteine residue near the C-terminus (from Wikipedia).

The modification can be used for the immobilization of the target protein.