Analysis of three human interleukin 5 structures suggests a possible receptor binding mechanism.

AbstractWe compared three crystal structures of human interleukin 5 (hIL5) expressed in either E. coli (hIL5E.coli), Sf9 cells (hIL5Sf9) or Drosophila cells (hIL5Drosophila). The dimeric hIL5 structures show subtle but significant conformational differences which are probably a consequence of the different crystallization conditions trapping this protein into one of two states. We refer to these two distinct conformations as the `open' and `tight' state, according to the packing around the cleft between the two subunits. We hypothesize that these two stable conformational states reflect the structure of the free or receptor bound hIL5

Characterization Of Functional Residues In The Interfacial Recognition Domain Of Lecithin Cholesterol Acyltransferase (Lcat)

Lecithin cholesterol acyltransferase (LCAT) is an interfacial enzyme active on both high-density (HDL) and low-density lipoproteins (LDL). Threading alignments of LCAT with lipases suggest that residues 50-74 form an interfacial recognition site and this hypothesis was tested by site-directed mutagenesis. The (delta56-68) deletion mutant had no activity on any substrate. Substitution of W61 with F, Y, L or G suggested that an aromatic residue is required for full enzymatic activity. The activity of the W61F and W61Y mutants was retained on HDL but decreased on LDL, possibly owing to impaired accessibility to the LDL lipid substrate. The decreased activity of the single R52A and K53A mutants on HDL and LDL and the severer effect of the double mutation suggested that these conserved residues contribute to the folding of the LCAT lid. The membrane-destabilizing properties of the LCAT 56-68 helical segment were demonstrated using the corresponding synthetic peptide. An M65N-N66M substitution decreased both the fusogenic properties of the peptide and the activity of the mutant enzyme on all substrates. These results suggest that the putative interfacial recognition domain of LCAT plays an important role in regulating the interaction of the enzyme with its organized lipoprotein substrates