The sterol carrier protein-2 (SCP2) is a 13.2 kDa basic protein that plays a multifunctional role in lipid metabolism in eukaryotic cells. Recent studies indicate that SCP2 functions as a ...non-specific lipid transfer protein and plays a key role in the peroxisomal β-oxidation of fatty acids and indirectly mediates downstream events in sterol biosynthesis. It has been suggested that SCP2 transports fatty acyl-CoA's to peroxisomes and mitochondria where they undergo β-oxidation. Consistent with this notion, SCP2 has a high affinity for the very long chain fatty acyl-CoA esters and their enoyl derivatives. The dimeric crystal structure of human SCP2 in its lipid free form was solved to 2.6 Å resolution using the method of multiple isomorphous replacement. The structure refined to an R-factor of 20.2% (Rfree = 24.1%). This structure consists of two layers. One layer is formed by a five-stranded mixed β-sheet and the second layer is formed by five α-helices. Analytical ultracentrifugation experiments showed that SCP2 binds to myristoyl-CoA with a 1:1 stoichiometry, and the SCP2 monomer dimerizes with a Kd = 1.88 mM. The structure of SCP2 provides an atomic model for understanding how activated fatty acids are transported. When human SCP2 is aligned and compared with monomeric crystal structure of rabbit SCP2, a hinged helix is revealed which could accommodate the wide variety and differing sizes of hydrophobic molecules to which SCP2 is known to bind. The conformational change in helix E upon dimerization causes an interaction with Lys 19, thus releasing the peroxisome targeting tri-peptide sequence from the protein surface.
Abstract Background The Fox gene family comprises a large and functionally diverse group of forkhead-related transcriptional regulators, many of which are essential for metazoan embryogenesis and ...physiology. Defining conserved functional domains that mediate the transcriptional activity of Fox proteins will contribute to a comprehensive understanding of the biological function of Fox family genes. Results Systematic analysis of 458 protein sequences of the metazoan Fox family was performed to identify the presence of the engrailed homology-1 motif (eh1), a motif known to mediate physical interaction with transcriptional corepressors of the TLE/Groucho family. Greater than 50% of Fox proteins contain sequences with high similarity to the eh1 motif, including ten of the nineteen Fox subclasses (A, B, C, D, E, G, H, I, L, and Q) and Fox proteins of early divergent species such as marine sponge. The eh1 motif is not detected in Fox proteins of the F, J, K, M, N, O, P, R and S subclasses, or in yeast Fox proteins. The eh1-like motifs are positioned C-terminal to the winged helix DNA-binding domain in all subclasses except for FoxG proteins, which have an N-terminal motif. Two similar eh1-like motifs are found in the zebrafish FoxQ1 and in FoxG proteins of sea urchin and amphioxus. The identification of eh1-like motifs by manual sequence alignment was validated by statistical analyses of the Swiss protein database, confirming a high frequency of occurrence of eh1-like sequences in Fox family proteins. Structural predictions suggest that the majority of identified eh1-like motifs are short α-helices, and wheel modeling revealed an amphipathicity that supports this secondary structure prediction. Conclusion A search for eh1 Groucho interaction motifs in the Fox gene family has identified eh1-like sequences in greater than 50% of Fox proteins. The results predict a physical and functional interaction of TLE/Groucho corepressors with many members of the Fox family of transcriptional regulators. Given the functional importance of the eh1 motif in transcriptional regulation, our annotation of this motif in the Fox gene family will facilitate further study of the diverse transcriptional and regulatory roles of Fox family proteins.
