'Granule enzymes' or 'granzymes' [1
] comprise about 90% of the mass of cytolytic granules, specialized 'secretory' lysosomes, of both cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. The granzymes are closely related structurally to chymotrypsin, with a triad of key residues - histidine, aspartic acid and serine - conserved at the catalytic site, and they are genetically linked to other leukocyte serine proteases, especially those of mast cells and monocytes. A total of eight granzymes (A-G and M) have been identified in the mouse, but only five are known in humans (A, B, H, M and tryptase-2, which is also known as granzyme 3). No human equivalents of mouse granzymes C-G are known, and granzyme H appears to be specifically human (Table ).
Properties of granzymes in humans and rodents
All granzyme genes are similarly organized, and their transcripts are made up from five exons, with the first encoding the leader sequence while exons 2, 3 and 5 encode individual amino acids of the catalytic triad (Figure ). Although most other granzyme genes encode only one transcript, two granzyme A mRNAs arise from alternative splicing of different exons 1.
Figure 1 Organization of the human granzyme B gene. The gene is approximately 3.2 kb in length and is presented as a prototype of granzyme gene organization. A TATA box consensus sequence (T) is present approximately 30 bp from the initiation site for transcription. (more ...)
Gene-mapping studies indicate that the three human and mouse granzyme subfamilies map to three corresponding loci, with each subfamily seeming to have a single, broad substrate specificity (Table ). Mapping of the rat genes has not been reported. The genes encoding human granzymes A (HFSP
) and tryptase-2 (TRYP2
) map to chromosome 5q11-q12 [2
]. Granzymes B and H map to a cluster of serine protease genes at 14q11, which includes the gene for the protease cathepsin G, which is specific for myeloid cells [3
]. The granzyme M gene is located with the genes encoding azurocidin (AZU
), neutrophil elastase (NE
) and proteinase-3 (PR3
) on chromosome 19p13.3 [2
]. In the mouse, the corresponding loci are chromosomes 10 for granzyme A and other tryptases, 14D (for granzymes B-F) and 10C (for granzyme M, AZU
Mouse and human granzymes grouped according to their chromosomal localization
Overall, the granzyme subfamilies have trypsin-like, chymotrypsin-like and elastase-like specificities, and their genes are grouped in the 'tryptase' locus, the 'chymase' locus and the 'Met-ase' locus, respectively (Table ) [4
]. These clusters of genes have their own peculiarities. For example, in mouse and human the granzyme M gene, AZU
each have intron 1 located between residues -7 and -6 of the leader sequence, indicating a close evolutionary relationship. In human, the genes encoding granzymes B and H and cathepsin G are very closely linked, mapping to within 50 kb of each other. The granzyme H gene is located between the other two genes, and appears to have arisen as a 'hybrid' made up of the first three exons and intervening introns of the granzyme B gene and the remainder of another serine protease gene. Generation of the hybrid might have been followed by the accumulation of point mutations.
Mouse, rat and human granzymes B are about 70% identical at the amino acid level, while granzymes within any one subfamily (for example, granzymes B and C in the mouse) are 55-70% identical. By contrast, amino-acid sequence identity drops to approximately 30-40% when one compares granzymes from different subfamilies, such as granzymes A and B (37%), even within the same species. Although granzymes probably occur in other species that have complex immune systems (for example, in birds), unfortunately to date granzyme sequences have only been reported in mammals.