UCP1 and UCP3 are members of the uncoupling protein (UCP) subfamily and are 
localized in the inner mitochondrial membrane. Whereas UCP1's central role in 
non-shivering thermogenesis is acknowledged, the function and even tissue 
expression pattern of UCP3 are still under dispute. Because UCP3 properties 
regarding transport of protons are qualitatively identical to those of UCP1, its 
expression in brown adipose tissue (BAT) alongside UCP1 requires justification. 
In this work, we tested whether any correlation exists between the expression of 
UCP1 and UCP3 in BAT by quantification of protein amounts in mouse tissues at 
physiological conditions, in cold-acclimated and UCP1 knockout mice. 
Quantification using recombinant UCP3 revealed that the UCP3 amount in BAT 
(0.51ng/(μg total tissue protein)) was nearly one order of magnitude higher than 
that in muscles and heart. Cold-acclimated mice showed an approximate three-fold 
increase in UCP3 abundance in BAT in comparison to mice in thermoneutral 
conditions. Surprisingly, we found a significant decrease of UCP3 in BAT of UCP1 
knockout mice, whereas the protein amount in skeletal and heart muscles remained 
constant. UCP3 abundance decreased even more in cold-acclimated UCP1 knockout 
mice. Protein quantification in UCP3 knockout mice revealed no compensatory 
increase in UCP1 or UCP2 expression. Our results do not support the 
participation of UCP3 in thermogenesis in the absence of UCP1 in BAT, but 
clearly demonstrate the correlation in abundance between both proteins. The 
latter is important for understanding UCP3's function in BAT.