Protein Arginine Deiminases and Associated Citrullination: Physiological Functions and Diseases Associated with Dysregulation

Abstract

Human proteins are subjected to more than 200 known post-translational modifications (PTMs) (e.g., phosphorylation, glycosylation, ubiquitination, S-nitrosylation, methylation, Nacetylation, and citrullination) and these PTMs can alter protein structure and function with consequent effects on the multitude of pathways necessary for maintaining the physiological homeostasis. When dysregulated, however, the enzymes that catalyze these PTMs can impact the genesis of countless diseases. In this review, we will focus on protein citrullination, a PTM catalyzed by the Protein Arginine Deiminase (PAD) family of enzymes. Specifically, we will describe the roles of the PADs in both normal human physiology and disease. The development of PAD inhibitors and their efficacy in a variety of autoimmune disorders and cancer will also be discussed.

Fig. 1. The process of citrullination

The primary amine group of peptidyl-arginine is hydrolyzed upon interaction with the cysteine of the deiminating enzyme and is replaced to form a keto group. The end products of this reaction are peptidyl-citrulline and free ammonium.

Fig. 2. Human PAD isozyme sequence alignments

Amino acid sequence alignment of the PAD isozymes with approximately aa 1–300 and 301–663 containing the N-terminus and C-terminus domains, respectively. Highlighted in black are amino acids that are conserved in all 5 isozymes, while gray highlighted amino acids are conserved in 4 of the 5 isozymes. In PADs 1–4, the important cysteine residue (Cys645) in the active site is highlighted in green and, in PAD4, the nuclear localization signal (NLS) sequence is highlighted in blue.

Fig. 3. Mechanisms of gene regulation involving PADs

A) Activated PAD4 citrullinates ING4 at the nuclear localization signal (NLS) region. Therefore, the regulatory domain (RD) of p53 cannot bind to the NLS of ING4, leading to inhibition of p53 acetylation and inhibition of subsequent upregulation of p21. B) PAD4 interacts with the regulatory domain of p53. The PAD4 attached to p53 is recruited to the promoter region (PR) of p21 where PAD4 citrullinates methylated arginine on histones at the promoter region. This citrullination, along with HDAC2, inhibits p21 transcription, resulting in an increase in cell proliferation and inhibition of cell cycle arrest. C) PAD4 citrullinates methylated arginine on the GRIP1 binding domain (BD) of p300. This leads to increased binding to GRIP1 and results in increased ER-mediated transcription.

Fig. 4. Structures of PAD inhibitors F-amidine and Cl-amidine

A) F-amidine and B) Cl-amidine share similar structures to arginine. Also these compounds are positive H bond donors that can sterically fit in the active site of PADs.