Intrinsically disordered N-terminus of calponin homology-associated smooth muscle protein (CHASM) interacts with the calponin homology domain to enable tropomyosin binding

Structural Characteristics, Binding Partners and Related Diseases of the Calponin Homology (CH) Domain

The calponin homology (CH) domain is one of the most common modules in various actin-binding proteins and is characterized by an α-helical fold. The CH domain plays important regulatory roles in both cytoskeletal dynamics and signaling. The CH domain is required for stability and organization of the actin cytoskeleton, calcium mobilization and activation of downstream pathways. The CH domain has recently garnered increased attention due to its importance in the onset of different diseases, such as cancers and asthma. However, many roles of the CH domain in various protein functions and corresponding diseases are still unclear. Here, we review current knowledge about the structural features, interactome and related diseases of the CH domain.

Binding of smoothelin-like 1 to tropomyosin and calmodulin is mutually exclusive and regulated by phosphorylation

Background

The smoothelin-like 1 protein (SMTNL1) can associate with tropomyosin (Tpm) and calmodulin (CaM), two proteins essential to the smooth muscle contractile process. SMTNL1 is phosphorylated at Ser301 by protein kinase A during calcium desensitization in smooth muscle, yet the effect of SMTNL1 phosphorylation on Tpm- and CaM-binding has yet to be investigated.

Results

Using pull down studies with Tpm-Sepharose and CaM-Sepharose, we examined the interplay between Tpm binding, CaM binding, phosphorylation of SMTNL1 and calcium concentration. Phosphorylation greatly enhanced the ability of SMTNL1 to associate with Tpm in vitro; surface plasmon resonance yielded a 10-fold enhancement in K_D value with phosphorylation. The effect on CaM binding is more complex and varies with the availability of calcium.

Conclusions

Combining both CaM and Tpm with SMTNL1 shows that the binding to both is mutually exclusive.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-017-0080-6) contains supplementary material, which is available to authorized users.

The C terminus of the catalytic domain of type A botulinum neurotoxin may facilitate product release from the active site

Botulinum neurotoxins are the most toxic of all compounds. The toxicity is related to a poor zinc endopeptidase activity located in a 50-kDa domain known as light chain (Lc) of the toxin. The C-terminal tail of Lc is not visible in any of the currently available x-ray structures, and it has no known function but undergoes autocatalytic truncations during purification and storage. By synthesizing C-terminal peptides of various lengths, in this study, we have shown that these peptides competitively inhibit the normal catalytic activity of Lc of serotype A (LcA) and have defined the length of the mature LcA to consist of the first 444 residues. Two catalytically inactive mutants also inhibited LcA activity. Our results suggested that the C terminus of LcA might interact at or near its own active site. By using synthetic C-terminal peptides from LcB, LcC1, LcD, LcE, and LcF and their respective substrate peptides, we have shown that the inhibition of activity is specific only for LcA. Although a potent inhibitor with a Ki of 4.5 μm, the largest of our LcA C-terminal peptides stimulated LcA activity when added at near-stoichiometric concentration to three versions of LcA differing in their C-terminal lengths. The result suggested a product removal role of the LcA C terminus. This suggestion is supported by a weak but specific interaction determined by isothermal titration calorimetry between an LcA C-terminal peptide and N-terminal product from a peptide substrate of LcA. Our results also underscore the importance of using a mature LcA as an inhibitor screening target.

The most important thing is the tail: multitudinous functionalities of intrinsically disordered protein termini

Many functional proteins do not have well-folded structures in their substantial parts, representing hybrids that possess both ordered and disordered regions. Disorder is unevenly distributed within these hybrid proteins and is typically more common at protein termini. Disordered tails are engaged in a wide range of functions, some of which are unique for termini and cannot be found in other disordered parts of a protein. This review covers some of the key functions of disordered protein termini and emphasizes that these tails are not simple flexible protrusions but are evolved to serve.