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The globular 25 kDa protein cytosine monophosphate kinase (CMPK,  EC ID: 2.7.4.14 ) from  E. coli  belongs to the family of nucleoside monophosphate (NMP) kinases (NMPK). Many proteins of this family share medium to high sequence and high structure similarity including the frequently found α/β topology. A unique feature of CMPK in the family of NMPKs is the positioning of a single  cis -proline residue in the CORE-domain ( cis -Pro124) in conjunction with a large insert in the NMP binding domain. This insert is not found in other well studied NMPKs such as AMPK or UMP/CMPK. We have analyzed the folding pathway of CMPK using time resolved tryptophan and FRET fluorescence as well as CD. Our results indicate that unfolding at high urea concentrations is governed by a single process, whereas refolding in low urea concentrations follows at least a three step process which we interpret as follows: Pro124 in the CORE-domain is in  cis  in the native state (N c ) and equilibrates with its  trans -isomer in the unfolded state (U c  - U t ). Under refolding conditions, at least the U t  species and possibly also the U c  species undergo a fast initial collapse to form intermediates with significant amount of secondary structure, from which the  trans -Pro124 fraction folds to the native state with a 100-fold lower rate constant than the  cis -Pro124 species. CMPK thus differs from homologous NMP kinases like UMP/CMP kinase or AMP kinase, where folding intermediates show much lower content of secondary structure. Importantly also unfolding is up to 100-fold faster compared to CMPK. We therefore propose that the stabilizing effect of the long NMP-domain insert in conjunction with a subtle twist in the positioning of a single  cis -Pro residue allows for substantial stabilization compared to other NMP kinases with α/β topology.

Folding Properties of Cytosine Monophosphate Kinase from E. coli Indicate Stabilization through an Additional Insert in the NMP Binding Domain