Mammals have special stores of fat—called brown adipose tissue—that help regulate body temperature. In brown fat cells, heat is generated by structures called mitochondria. But mitochondria rarely act alone; in most cells, they are physically and functionally coupled to a network of membranes, called the endoplasmic reticulum (ER), which plays a key role in protein folding, quality control, and cellular signaling. In preliminary studies, I found that mice lacking a particular protein in the ER of fat cells were unable to maintain their body temperature following exposure to the cold. Using a suite of techniques in biochemistry, physiology, and cell and molecular biology, I will determine exactly how this deficit in the ER affects the ability of mitochondria to produce heat in brown fat—work that could lead to novel treatments for obesity.