Lipase (LipA) / lipase chaperon (Lif) complex
Burkholderia glumae (bacteria)
Molecular chaperones help other proteins to fold, which is essential for the proteins' function. There are two kinds of molecular chaperones, steric chaperones and non-steric chaperones. Steric chaperones help correct folding by lowering the energy barrier between the native (folded) state and partially folded states and imprinting specific structural information onto the target protein. On the other hand, non-steric chaperones does their work simply by inhibiting unfavorable side reactions for correct folding without imposing any target-specific structural information.
The widely conserved type II secretion system in Gram-negative bacteria is one of the system involving steric chaperones. In this secretion system, many virulence factors including a lipase called LipA were transported across the outer membrane. During the secretion, a lipase-specific foldase Lif helps LipA to obtain active and competent conformation in the periplasm. Without the help of Lif, lipase is not secreted and folds into an inactive intermediate form.
The structure shown here is the complex of Lif-LipA from Bulkholderia glumae. Lif consists of 11 alpha-helices, which engulfs the lipase. Two globular minidomains (alpha1-3 and alpha9-11, respectively) at the N and C termini are connected with a flexible extended alpha-helical motif (alpha4-8). There is a well-conserved sequence motif in alpha-1 helix in N-terminal globular mini-domain, which is essential for activation of LipA. Lif is considerably more flexible than LipA, and the structure of LipA is almost as same as native Lif-free form which suggest that structural change upon binding occurs on the side of Lif rather than LipA.
From the past UV CD spectroscopy experiment and the present structural analysis, the authors of this paper suggest that the formation of one or more alpha-helices in the extended alpha-helical motif in Lif occurs upon binding, and helps lipase to fold into its native conformation by providing a folding platform.
The structure shown here includes two sets of Lif/LipA complexes. The figure 1 displays the structure of one pair of Lif/LipA complex.
Figure 1: Lipase chaperone (Lif) complexed with lipase (LipA)
Protein Data Bank (PDB)
author: Miho Higurashi