Calmodulin
(CaM) regulates numerous cellular functions by sensing Ca2+ levels inside cells. Although its structure as a function of the
Ca2+-bound state remains hotly debated, no report is available
on how pH independently or in interaction with Ca2+ ions
regulates shape and function of CaM. From SAXS data analysis of CaM
at different levels of Ca2+-ion concentration and buffer
pH, we found that (1) CaM molecules possess a Gaussian-chain-like
shape in solution even in the presence of Ca2+ ion or at
low pH, (2) the global shape of apo CaM is very similar to its NMR
structure rather than the crystal structures, (3) about 16 Ca2+ ions or more are required per CaM molecule in solution to
achieve the four-Ca2+-bound crystal structure, (4) low
pH alone can impart shape changes in CaM similar to Ca2+ ions, and (5) at different [Ca2+]/[CaM] ratio or pH values,
the predominant shape of CaM is essentially a weighted average of
its apo and fully activated shape. Results were further substantiated
by analysis of sedimentation coefficient values from analytical ultracentrifugation
and peptide binding assays using two peptides, each known to preferentially
bind the apo or the Ca2+-activated state.
Background: It remains unaddressed whether varying neutralizing potency of mAbs is somehow correlated with differences in their global shapes. Results: Non-neutralizing mAbs have an open shape, whereas Fab-Fab and Fab-Fc interactions induce a closed shape in HIV-1-neutralizing mAbs.
Conclusion:An unopen shape appears to be a hallmark of neutralizing potency, at least for HIV-1. Significance: This work provides new insight into the shape-function relationship of mAbs.
Dimerization of bacterial chaperone trigger factor (TF) is an inherent protein concentration based property which available biophysical characterization and crystal structures have kept debatable. We acquired small-angle X-ray scattering (SAXS) intensity data from different TF homologues from Escherichia coli (ECTF), Vibrio cholerae (VCTF), and Psychrobacter f rigidicola (PFTF) while varying each protein concentration. We found that ECTF and VCTF adopt a compact dimeric shape at higher concentrations which did not resemble the "back-to-back" conformation reported earlier for ECTF from crystallography (PDB ID: 1W26). In contrast, PFTF remained monomeric throughout the concentration range 2−90 μM displaying a multimodal open extended conformation. OLIGOMER analysis showed that both the ECTF and VCTF remained completely monomeric at lower concentrations (2−11 μM), while, at higher concentrations (60−90 μM), they adopted a dimeric form. Interestingly, the equilibrium existed in the medium concentration range (>11 and <60 μM), which correlates with the physiological concentration (40−50 μM) of TF in cell cytoplasm. Additionally, circular dichroism data revealed that solution structures of ECTF and VCTF contain predominantly αhelical content, while PFTF contains 3 10 -helical content.
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