A vector for expressing a soluble glutaminyl cyclase (QC) in bacterial cells is described. The vector contains a sequence encoding a fusion QC, which has a fusion protein tag selected from a Nus protein tag or a thioredoxin (Trx) tag, a QC, and a linker having at least one (His)x-tag between the QC and the fusion protein tag, in which x is an integer of at least 6. Methods for expressing a soluble glutaminyl cyclase (QC) by the vector are also described.