Conformation of bovine nerve root P2 protein: characteristics of the molecule from circular dichroism spectra

Bovine P2 myelin basic protein crystallizes in three different forms

P2 protein is a minor component of the myelin membrane. We have crystallized this protein for high-resolution crystallographic study. Three crystal morphologies are available. Two of them are from ammonium sulfate, and one is from polyethyleneglycol (PEG). The unit cell of the most suitable crystals from PEG 4000 has the dimensions a = 91.3 A, b = 99.8 A, c = 56.0 A; is of space group P2(1)2(1)2(1); and contains up to four molecules per asymmetric unit. The limit of resolution is 2.7 A.

Reconstituted P2/myelin-lipid multilayers

A complex forms when bovine P2 protein is added to single-bilayer vesicles created by sonicating myelin lipids. The complex was studied by biochemical analysis, freeze-fracture (FF) and thin-section electron microscopy (EM), and by X-ray diffraction. Smaller amounts of P2 cause the vesicles to aggregate and fuse whereas larger amounts (greater than or equal to 4 wt%) cause multilayers to form. Binding saturates at 15 wt% P2. FF EM shows that large, flat multilayers form within 15 min of addition of P2. Only smooth fracture faces are seen, as expected for a peripheral membrane protein. X-ray diffraction shows a constant repeating distance in the multilayers: 86.0 +/- 0.7 A between the centers of bilayers in the range 4 wt% less than or equal to P2/(P2 + lipid) less than or equal to 15 wt%. Assuming a 53 A-thick bilayer, the space between bilayers is 33 A wide. This is a wider space than for myelin basic protein (MBP) (20-25 A wide). The respective widths are consistent with a compact, globular structure for P2 and a flattened shape for MBP. Calculated electron-density profiles of the lipids with and without P2 reveal the protein largely in the interbilayer spaces, with a small part possibly inserted into the lipid headgroup layers. The different proportions of P2 in the sciatic nerve of various species are tentatively correlated with the different average widths observed by X-ray diffraction for the cytoplasmic space (major period line) between bilayers in the respective sciatic myelins.

Degradation of bovine P2 protein by bovine brain cathepsin D

Bovine brain cathepsin D cleaved bovine P2 protein to produce three major and several minor peptides. The major P2 peptides formed were shown by amino acid analysis and partial sequencing to be peptides 17-54, 20-58 and 65-131 with the latter predominating. In preliminary experiments, P2 peptide 65-131 did not induce experimental allergic neuritis in Lewis rats in equimolar amounts to the neuritogenic P2.

Immune responses to myelin antigens in Guillain-Barré syndrome

Antibodies to nerve antigens were sought in the sera of 17 patients with acute Guillain-Barré syndrome (GBS), 11 with chronic relapsing demyelinating poly-radiculoneuropathy (CRP), 20 with other neuropathies (ON), 15 with other neurological diseases (OND) and 19 normal subjects. Complement-fixing antibodies to a suspension of human peripheral nerve tissue were identified in only 2 patients with GBS and 1 with chronic progressive neuropathy. Five GBS sera gave complement fixation reactions with rabbit sciatic nerve. The sera were also tested for galactocerebroside (Gal-C) binding activity using a solid phase assay. The range of values in all groups was the same, although the mean values for patients with GBS, ON and OND were higher than those of normal subjects. In a radioimmunoassay for antibodies to bovine P2 slightly more radiolabelled antigen was precipitated by the GBS group of sera than by sera from the other groups, but only one serum from the GBS and another from the CRP patients precipitated more than 10% of the label. Addition of bovine P2 to cultures of peripheral blood mononuclear cells from 11 patients with GBS did not cause significant stimulation. Immunoassay for antibody to myelin basic protein (MBP) showed an increased proportion of sera with low binding activity in the GBS and CRP groups. The results suggest that humoral immune responses to potentially neuritogenic antigens are found with marginally increased frequency in patients with GBS and CRP.

A general model of the P2 protein of peripheral nervous system myelin based on secondary structure predictions, tertiary folding principles, and experimental observations

The amino acid sequence of the P2 protein of peripheral myelin was analyzed with regard to regions of probable alpha-helix, beta-structure, beta-turn, and unordered conformation by means of several algorithms commonly used to predict secondary structure in proteins. Because of the high beta-sheet content and virtual absence of alpha-helix shown by the circular dichroic spectra of the protein, a bias was introduced into the algorithms to favor the beta-structure over the alpha-helical conformation. In order to define those beta-sheet residues that could lie on the external hydrophilic surface of the protein and those that could lie in its hydrophobic interior, the predicted beta-strands were examined for charged and uncharged amino acids located at alternating positions in the sequence. The sequential beta-strands in the predicted secondary structure were then ordered into beta-sheets and aligned according to generally accepted tertiary folding principles and certain chemical properties peculiar to the P2 protein. The general model of the P2 protein that emerged was a "Greek key" beta-barrel, consisting of eight antiparallel beta-strands with a two-stranded ribbon of antiparallel beta-structure emerging from one end. The model has an uncharged, hydrophobic core and a highly hydrophilic surface. The two Cys residues, which form a disulfide, occur in a loop connecting two adjacent antiparallel strands. Two hydrophilic loops, each containing a cluster of acidic residues and a single Phe, protrude from one end of the molecule. The general model is consistent with many of the properties of the actual protein, including the relatively weak nature of its association with myelin lipids and the positions of amino acid substitutions. Alternative beta-strand orderings yield three specific models having different interstrand connections across the barrel ends.