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The CBM19 found in the CTS1 chitinase produced by Saccharomyces cerevisiae has been characterized as a chitin-binding CBM [1]. The authors produced several manipulated constructs in yeast to demonstrate chitin binding including a C-terminal deletion mutant, controlled proteolysis of the wild-type enzyme, production of a fused N-terminal signal sequence directly to the C-terminal CBM19 and fusion of the CBM19 with the yeast invertase SUC2 [1]. These different constructs were then tested for their chitin-binding capabilities. The CBM19 is suggested to bind with high affinity, though no binding affinity was experimentally determined [1]. Interestingly, the C-terminal deletion mutant showed an enhanced rate of chitin hydrolysis relative to the wild type.
Structural Features
Figure 1. A primary structure representation of the S. cerevisiae CST1 protein (not to scale)[1]. The N and the C termini are annotated. SP represents signal peptide, GH18 is a family 18 glycoside hydrolase, S/T rich is the serine/threonine-rich heavily glycosylated region and CBM19 is the family 19 carbohydrate-binding module.
Two alleles of the cts1 gene have been identified, they are referred to as cts1-1 and cts1-2 [1]. The predicted full length protein is likely divided into four domains (Figure 1) [1]. The first domain (amino acids 1-20) is the cleavable signal sequence, the second domain is the chitinase GH18 catalytic domain (amino acids 21-327), the third domain is a highly glycosylated serine/threonine-rich domain (amino acids 328-480) and the fourth domain is the chitin binding CBM19 [1, 2].
There is no 3D structure available for the CBM19 family.
Functionalities
Chitin is an important component of the cell wall of S. cerevisiae. It is specifically located at the junction of mother and daughter cells providing mechanical stability. The CTS1 enzyme, containing a GH18 catalytic module is produced by S. cerevisiae and hydrolyses chitin[3, 4] and has a role in cell separation [1]. When chitinase activity was disrupted cells were unable to separate normally resulting in aggregates of cells connected by their cell septum regions [1]. Complementation experiments were able to restore the separation phenotype with plasmids containing cts1-1 or cts1-2; however, the C-terminal deletion mutant only partly restores the phenotype [1]. This suggests the CBM19 is involved in the mechanism of cell separation between mother and daughter cells.
Family Firsts
First Identified
The chitin-binding CBM19 from CTS1 in Saccharomyces cerevisiae was first described by Kuranda and Robbins in 1991 [1].
First Structural Characterization
There is no 3D structural data on the CBM19 family.
