Required for proper prespore cell patterning. Plays a role in stabilizing mkkA by preventing it from being targeted for degradation. ubcB and ubpB differentially control ubiquitination/deubiquitination and degradation of mkkA in a cell-type-specific and temporally regulated manner. Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Interacts with mkkA (via F-box/WD40 domains). Found at low levels in vegetative cells and at higher levels during aggregation and mound formation (4 and 8 hours of development). Null cells differentiate into prestalk cells preferentially. They develop precociously and exhibit abnormal cell-type patterning with an increase in the prestalk domain and reduction in the prespore domain in the slug, and spore domain in a fruiting body. Null cells do not form the normal spherical sori seen in mature wild-type fruiting bodies. Instead, the sori remain elongated, similar to those seen in wild-type fruiting bodies that are not fully mature. The upper and lower cups of the fruiting body, which are derived from prestalk cells and anterior-like cells, are enlarged. The spore-containing region is reduced. Null cells form fruiting bodies at approximately 17 hours and exhibit precocious expression of the spore marker. The expression levels of the prespore/spore markers are lower in null cells compared to wild-type cells, whereas expression of the prestalk/stalk marker is elevated slightly. The loss of gene function, might lead to increased mkkA ubiquitination and a more rapid turnover of mkkA. This should result in a reduced level of mkkA activity, producing the above mentioned mkkA null cell type phenotype. Belongs to the peptidase C19 family.