Biology subjects

Rohaim, A.

Publications and source records attributed to Rohaim, A..

3 recordsLinked to original sources

bioRxiv2021

A different mechanism of C-type inactivation in the Kv-like KcsA mutant E71V

A large class of K+ channels display a time-dependent phenomenon called C-type inactivation whereby prolonged activation by an external stimulus leads to a non-conductive conformation of the selectivity filter. C-type inactivation is of great physiological importance particularly in voltage-activated K+ channels (Kv), affecting the firing patterns of neurons and shaping cardiac action potentials. While understanding the molecular basis of inactivation has a direct impact on human health, its structural basis remains unresolved. Knowledge about C-type inactivation has been largely deduced from the pH-activated bacterial K+ channel KcsA, whose selectivity filter under inactivating conditions adopts a constricted conformation at the level of the central glycine (TTVGYGD) that is stabilized by tightly bound water molecules. However, C-type inactivation is highly sensitive to the molecular environment surrounding the selectivity filter in the pore domain, which is different in Kv channels than in the model KcsA. In particular, a glutamic acid residue at position 71 along the pore helix in KcsA is consistently substituted by a nonpolar valine in most Kv channels, suggesting that this side chain is an important molecular determinant of function. Here, a combination of X-ray crystallography, solid-state NMR and molecular dynamics simulations of the E71V mutant of KcsA is undertaken to explore the features associated with this Kv-like construct. In both X-ray and ssNMR data, it is observed that the filter of the Kv-like KcsA mutant does not adopt the familiar constricted conformation under inactivating conditions. Rather, the filter appears to adopt a conformation that is slightly narrowed and rigidified over its entire length. No structural inactivation water molecules are present. On the other hand, molecular dynamics simulations indicate that the familiar constricted conformation can nonetheless be stably established in the mutant channel. Together, these findings suggest that the Kv-like E71V mutation in the KcsA channel may be associated with different modes of C-type inactivation, showing that distinct selectivity filter environments entail distinct C-type inactivation mechanisms.

molecular biology↗

bioRxiv2021

Quaternary structure independent folding of voltage-gated ion channel pore domain subunits

Every voltage-gated ion channel (VGIC) superfamily member has an ion conducting pore consisting of four pore domain (PD) subunits that are each built from a common plan comprising an antiparallel transmembrane helix pair, a short, obliquely positioned helix (the pore helix), and selectivity filter. The extent to which this structure, the VGIC-PD fold, relies on the extensive quaternary interactions observed in PD assemblies is unclear. Here, we present crystal structures of three bacterial voltage-gated sodium channel (BacNaO_SCPLOWVC_SCPLOW) pores that adopt a surprising set of non-canonical quaternary structures and yet maintain the native tertiary structure of the PD monomer. This context-independent structural robustness demonstrates that the VGIC-PD fold, the fundamental VGIC structural building block, can adopt its native-like tertiary fold independent of native quaternary interactions. In line with this observation, we find that the VGIC-PD fold is not only present throughout the VGIC superfamily and other channel classes but has homologs in diverse transmembrane and soluble proteins. Characterization of the structures of two synthetic Fabs (sFabs) that recognize the VGIC-PD fold shows that such sFabs can bind purified full-length channels and indicates that non-canonical quaternary PD assemblies can occur in the context of complete VGICs. Together, our data demonstrate that the VGIC-PD structure can fold independently of higher-order assembly interactions and suggest that full-length VGIC PDs can access previously unknown non-canonical quaternary states. These PD properties have deep implications for understanding how the complex quaternary architectures of VGIC superfamily members are achieved and point to possible evolutionary origins of this fundamental VGIC structural element.

biophysics↗

bioRxiv2021

Crystal structure of a bacterial CNNM magnesium transporter

CBS-pair domain divalent metal cation transport mediators (CNNMs) are a broadly conserved family of integral membrane proteins with close to 90,000 protein sequences known. CNNM proteins are associated with Mg2+ transport but it is not known if they mediate transport themselves or regulate other transporters. Here, we determined the crystal structure of an archaeal CNNM protein with Mg2+-ATP bound. The structure reveals a novel transmembrane fold for the DUF21 domain, the largest family of domains of unknown function. The protein has a negatively charged cavity that penetrates halfway through the membrane suggesting it functions as a cation transporter. The cytosolic portion of the protein is comprised of highly charged four-helix bundle and a CBS-pair domain. HDX-MS experiments, molecular dynamics, and additional crystal structures show that the cytosolic domains undergo large conformational changes upon nucleotide binding suggesting a mechanism of regulation shared between human and bacterial orthologs. The molecular characterization of CNNM proteins has profound implications for understanding their biological functions in human diseases, including cancer, and in animals, bacteria and plants.

biophysics↗