Abstract
The 30-kDa family of lipoproteins from insect hemolymph has been the focus of a number of studies over the last few years. Recently, four crystal structures of Bombyx mori lipoprotein 7 have been determined. Here we report two crystal structures of another member of the 30-kDa lipoprotein family, Bombyx mori lipoprotein 3 (Bmlp3). The protein was isolated from its natural source, mulberry silkworm hemolymph. It crystallized in two different crystal forms, Bmlp3-p21 (space group P21) and Bmlp3-c2 (space group C2). The crystal structures were solved by molecular replacement using the coordinates of Bmlp7 as a starting model. The crystals of Bmlp3-p21 diffracted X-rays to 2.4 Å resolution and of Bmlp3-c2 to 2.1 Å resolution. Bmlp3 has an overall fold characteristic of 30-kDa lipoproteins, with a VHS-type N-terminal domain and β-trefoil C-terminal domain. Structural comparison of Bmlp3 and Bmlp7 shows that the loops present in the C-terminal domain are flexible and participate in dimer formation. Additionally, new putative binding sites of Bmlp3 have been analyzed in detail and the electrostatic potential of the protein surface at physiological pH 7.4 conditions has been calculated. The results of these calculations are the starting point for an explanation of the recently reported cell-penetrating properties of the 30-kDa lipoproteins.
Highlights
Mulberry silkworm, Bombyx mori, was domesticated by humans at least five thousand years ago
We present the first crystal structures of Bombyx mori lipoprotein 3 (Bmlp3), which is another member of the 30-kDa lipoprotein family in B. mori
Our study reveals potential new binding sites in addition to those proposed earlier [22] and establishes a reliable picture of the electrostatic potential on the protein surface, which is of key importance for the understanding of its intermolecular interactions
Summary
Bombyx mori, was domesticated by humans at least five thousand years ago. Members of the 30-kDa lipoprotein family are the most abundant protein fraction in the hemolymph of silkworm fifth instar larvae [7]. According to the genomic sequence, ten genes (Bmlp1-Bmlp10) of 30-kDa lipoproteins were predicted Alignment of their deduced amino acid sequences as well as their phylogenetic analysis indicated that these proteins could be divided into three subfamilies. Members of 30-kDa lipoprotein family have been found in a number of other species of the Lepidoptera order, e.g. Samia cynthia ricini, Antheraea assama or Antheraea mylitta [14]. Before this genomic analysis was completed, only microvitellogenin isolated from Manduca sexta had been reported to be homologous to silkworm 30kDa lipoproteins [15].
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