Side Chain Mobility and Ligand Interactions of the G Strand of Tear Lipocalins by Site-Directed Spin Labeling

Abstract

Side chain mobility, accessibility, and backbone motion were studied by site-directed spin labeling of sequential cysteine mutants of the G strand in tear lipocalins (TL). A nitroxide scan between residues 98 and 105 revealed the alternating periodicity of mobility and accessibility to NiEDDA and oxygen, characteristic of a beta-strand. Residue 99 was the most inaccessible to NiEDDA and oxygen. EPR spectra with the fast relaxing agent, K(3)Fe(CN)(6), exhibited two nitroxide populations for most residues. The motionally constrained population was relatively less accessible to K(3)Fe(CN)(6) because of dynamic tertiary contact, probably with side chain residues of adjacent strands. With increasing concentrations of sucrose, the spectral contribution of the immobile component was greater, indicating a larger population with tertiary contact. Increased concentrations of sucrose also resulted in a restriction of mobility of spin-labeled fatty acids which were bound within the TL cavity. The data suggest that sucrose enhanced ligand affinity by slowing the backbone motion of the lipocalin. The correlation time of an MTSL derivative (I) attached to F99C resulted in the lack of side chain motion and therefore reflects the overall rotation of the TL complex. The correlation time of F99C in tears (13.5 ns) was the same as that in buffer and indicates that TL exists as a dimer under native conditions. TL-spin-labeled ligand complexes have a shorter correlation time than the protein alone, indicating that the fatty acids are not rigidly anchored in the cavity, but move within the pocket. This segmental motion of the ligand was modulated by protein backbone fluctuations. Accessibility studies with oxygen and NiEDDA were performed to determine the orientation and depth of a series of fatty acid derivatives in the cavity of TL. Fatty acids are oriented with the hydrocarbon tail buried in the cavity and the carboxyl group oriented toward the mouth. In general, the mobility of the nitroxide varied according to position such that nitroxides near the mouth had greater mobility than those located deep in the cavity. Nitroxides positioned up to 16 carbon units from the hydrocarbon tail of the ligand are motionally restricted and inaccessible, indicating the cavity extends to at least this depth. EPR spectra obtained with and without sucrose showed that the intracavitary position of lauric acid in TL is similar to that in beta-lactoglobulin. However, unlike beta-lactoglobulin, TL binds 16-doxyl stearic acid, suggesting less steric hindrance and greater promiscuity for TL.

Molten globule state of tear lipocalin: ANS binding restores tertiary interactions

“…These spectra are very similar to that of holo-forms that have been studied previously [27]. For some sites, two dynamic modes, indicated as α and β in Figure 1, can be easily observed.…”

“…All single Cys mutant and wild-type proteins used in this study have been characterized previously [27]. For G-strand residues of TL, the effects of mutations on secondary structure and binding characteristics are minimal.…”

“…It has been shown that the peak-to-peak intensity and line width of the central line of the EPR spectra correlate with dynamic and accessibility parameters. For β-sheet proteins, the sequence pattern of these site specific parameters along the β-strand exhibits characteristic alternating periodicity [24,27]. The nitroxide side chain in β-sheets that is located in a tertiary interaction with a side chain of a neighboring strand gives rise to additional dynamic modes, ranging from weakly ordered to immobilized.…”

“…For single Cys mutants of TL, C101L was chosen as the template for additional mutant cDNAs because it showed similar structural features and binding characteristics for spin labeled lipids [27]. Eight additional mutant cDNAs were constructed, in which corresponding amino acid residues in the G strand of TL were substituted sequentially to cysteine.…”

Evidence for internal and external binding sites on human tear lipocalin

“…Laser light scattering studies have confirmed that the recombinant protein is monomeric similar to the native protein [25]. Previously, cysteine labeling studies of the recombinant protein have confirmed that the thiol group on C101 was available for labeling, excluding the possibility of disulfide scrambling [26]. The labeling efficiency of the recombinant protein never exceeded a ratio of 1:1.…”