 The GUK domain that they have is structurally very similar to that of the guanylate kinases, however it is known to be catalytically inactive as the P-Loop which binds ATP is absent.It is thought that the MAGUKs have subfunctionalized the GUK domain for their own purposes, primarily based on its ability to form protein-protein interactions with cytoskeleton proteins, microtubule/actin based machinery and molecules involved in signal transduction.The PDZ domain which are contained in the MAGUKs in varying numbers, is replicated three times over in PSD-95.