Important bacterial pathogens manipulate eukaryotic target cells by injecting effector proteins through type III secretion systems (T3SS). Recent in situ observations revealed that these large molecular machines, also called injectisomes, are amazingly dynamic and adaptive entities. We recently found that the cytosolic components of the T3SS form a mobile network that shuttles effector proteins from the cytosol to the export machinery (Wimmi et al, Nature Microbiol 2024). In contrast to these soluble components, the transmembrane rings anchoring the injectisome are stably associated – with one exception. Using live cell microscopy, single particle tracking and functional assays, we found that SctD, which forms the inner membrane ring of the T3SS, exchanges subunits in secreting injectisomes. To decipher the biological function of the unexpected mobility of this central structural component of the T3SS, we analyzed its role in assembly and function of the injectisome. Based on molecular dynamics modeling, we engineered SctD to allow to tune its exchange rate. The experiments showed that lower exchange leads to decreased secretion of effectors, revealing a direct correlation between protein exchange and the function of the T3SS. Our findings uncover a new aspect of the molecular function and regulation of the T3SS, which may be applicable to other secretion systems and molecular machines.