Protein $A$ and protein $B$ form a covalent complex. Gel filtration chromatography of this complex showed a peak corresponding to $200 \:kDa$. $SDS-PAGE$ analysis of this complex, with and without beta-mercaptoethanol, showed a single band corresponding to molecular weight $50$ and $25 \:kDa$, respectively. Given that the molecular weight of protein $A$ is $25 \:kDa$, the molecular weight of protein $B$ is ________________ $kDa$.