Bulkiness of the DIV W[+2] residue tunes VDI. (A) Exemplar traces reveal intermediate inactivation of Ca_V1.3 DIV W[+2]T mutant (blue). Exemplar traces from wild-type (W[+2]) and DIV W[+2]A mutant channels are reproduced from Fig. 1 F to facilitate comparison. (B) VDI is similarly diminished for DIV W[+2]V mutation. Format as in panel A. (C) VDI is virtually absent for DIV W[+2]F mutation. (D) VDI is enhanced for DIV W[+2]G mutation. (E) Left: Structural model highlights residues in adjacent domains that are within 5 Å of W[+2] residue in a given domain. Right: Sequence alignment and residues in close vicinity of W[+2] residue. Neighborhood residues E[0], T[−2], and F[−6] are conserved across different domains. By contrast, DIV W[+2] is in the vicinity of DIII T[−5] while either a Q or an R is present at the −5 position of other domains. (F) Analysis of DIII T[−5]Q/DIV W[+2]A double mutation shows reduced VDI in comparison with DIV W[+2]A single mutation. (G) Population r₈₀₀ values for DIV W[+2] mutations reveal a Boltzmann relation between VDI as the accessible side-chain surface area. DIII T[−5]Q/DIV W[+2]A reduced VDI in comparison to DIV W[+2]A mutation and deviates from this relationship. Each symbol and error, mean ± SEM from n = 5 cells for each mutant.