Effect of citrulline for arginine replacement on the structure and turnover of phosphopeptide substrates of protein phosphatase-1

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Fluorescent peptide probes for high-throughput measurement of protein phosphatases

A homogeneous microplate assay for the serine/threonine protein phosphatases PP1 and PP2A, employing fluorescent-labeled phosphopeptides, has been developed. Phosphopeptides derived from a phosphoacceptor site in myelin basic protein were designed with a cysteine adjacent to the phosphoresidue, allowing site-selective labeling with dyes. The fluorescence emission from the environmentally sensitive fluorophore 7-fluorobenz-2-oxa-1,3-diazole-4-sulfonamide was found to be sensitive to the phosphorylation status of an adjacent threonine residue. Upon complete dephosphorylation of the dye-labeled phosphopeptide, a 56% decrease in fluorescence intensity was observed. The change in fluorescence was correlated with the release of inorganic phosphate from the phosphopeptide as measured using the malachite green assay. Conjugation of the fluorophore to the phosphopeptide was found to have no adverse effect on catalysis. A series of four phosphopeptide substrates were developed and characterized to probe PP1 and PP2A activity. The optimum phosphopeptides were then used to determine inhibition parameters for three natural protein phosphatase inhibitors. The use of a peptide-based approach has introduced a degree of specificity not observed with many conventional phosphatase substrates, while retaining the advantages of a real-time homogeneous fluorescence-based format, making the assay ideal for high-density screening.

Arginine to citrulline replacement in substrates of phosphorylase kinase

Synthetic peptides based on residues 9 to 18 of glycogen phosphorylase were prepared containing citrulline at position 10 or 16, or at both positions 10 and 16. The peptides were compared as substrates for a recombinant, truncated form of the catalytic subunit of phosphorylase kinase (residues 1-300). The peptide having citrulline at position 10 was phosphorylated the same as the parent peptide. Both the peptides with a single citrulline at position 16 and with two citrullines were phosphorylated less effectively than the parent peptide; k(cat)/K(m) values were approximately 20% the value with the parent peptide. Incorporation of the second citrulline had little change in the effectiveness of the peptide as a substrate although the kinetic parameters with the citrulline peptides did show differences. The change in peptide phosphorylation did not seem to result from a change in peptide structure. Two-dimensional nuclear magnetic resonance studies of di-citrulline peptide are reported and showed no change in the solution structure of the peptide compared to the parent peptide. Thus, the change in kinetic parameters with the modified peptides seemed an effect of arginine replacement and was likely a consequence of the loss of charge inasmuch as the size of arginine and citrulline are similar. Arginine-16 was concluded to be more important for phosphorylase kinase recognition than arginine-10. These findings were consistent with the earlier studies using alanine replacement of arginine in synthetic peptides as substrates for the holoenzyme form of phosphorylase kinase.