T cell homeostasis is critical for the proper function of the immune system. Following the sharp expansion upon pathogen infection, most T cells die in order to keep balance in the immune system, a process which is controlled by death receptors during the early phase and Bcl-2 proteins in the later phase. It is still highly debated whether the apoptosis of T cells is determined from the beginning, upon activation, or determined later during the contraction. MCL1, a Bcl-2 family member, plays a pivotal role in T cell survival. As a fast turnover protein, MCL1 levels are tightly regulated by the 26S proteasome-controlled protein degradation process. In searching for regulatory factors involved in the actions of MCL1 during T cell apoptosis, we found that ALG-2 was critical for MCL1 stability, a process mediated by a direct interaction between ALG-2 and Rpn3, a key component of the 26S proteasome. As a critical calcium sensor, ALG-2 regulated the activity of the 26S proteasome upon increases to cytosolic calcium levels following T cell activation, this consequently influenced the stability of MCL1 and accelerated the T cell "death" process, leading to T cell contraction and restoration of immune homeostasis. Our study provides support for the notion that T cells are destined for apoptosis after activation, and echoes the previous study about the function of ALG-2 in T cell death.