Preface [Hot Topic: Gingipains (Guest Editor: Kenji Yamamoto)]

Gingipains are a novel class of cysteine proteinases produced by Porphyromonas gingivalis, a Gram-negative, black-pigmented, asaccharolytic, and anaerobic bacterium, which is implicated as a major etiologic agent in certain forms of periodontitis, particularly adult periodontitis. Gingipains are classified into two types of proteinases based on their peptide bond cleavage specificity. One is arginine-X-specific cysteine proteinases (Arg-gingipains, gingipain-Rs, Rgps), and the other is lysine-X-specific cysteine proteinase (Lys-gingipain, gingipain-K, Kgp). Rgps are encoded by two separate rgp genes (rgpA and rgpB), whereas Kgp is encoded by a single kgp gene. The majority of both Rgps and Kgp is associated with the bacterial membranes and the minority is secreted extracellularly. There is no doubt that efforts to obtain a deeper insight into the action of these enzymes were stimulated, not only by the general trend towards molecular-level examination in bacteriology and biochemistry, but also by new concept of bacterial proteinases as possible therapeutic targets. Gingipains can degrade a wide range of host proteins independently or cooperatively and thereby contribute to the virulence of the bacterium. On the other hand, both enzymes are essential for the bacterium to proliferate and survive in the periodontal pockets. These findings clearly show that gingipains participate in diverse pathological processes in humans and various physiological regulatory mechanisms in the bacterium. Recent studies using various P. gingivalis mutants deficient in Rgp- and / or Kgp-encoding genes reveal that most of the virulence of the bacterium is attributable to both Rgp and Kgp. Thus, recent advance in understanding the role of gingipains under physiological and pathological conditions are valuable focal points in the biological and medical fields. Since the gingipains world is expanding very rapidly, it is impossible to cover all the new aspects of this field. However, this issue includes manuscripts on molecular and structural characteristics, biological functions, processing of gingipains and their inhibitors. The editor hopes that the novel ideas, approaches, methodologies, and important findings described in this issue will give a new idea of the current status, trends, and directions of the field and that this publication will provide an impetus for further relevant research areas. Finally, the editor wishes to express special thanks to those who contributed to the publication of this special issue.