Table 1.

Molecular weights of the circadian proteins and complexes

Complex	Calculation	Mass	Components
PER–CRY–CK1	4205∗15.6∗10.79 4205∗7.9∗7.7	707 kDa	PER1-3, CRY1-2, CK1 CLOCK–BMAL1
CLOCK/BMAL1		255 kDa
PER1		136 kDa
PER2		137 kDa
PER3		132 kDa
CRY1		66 kDa
CRY2		67 kDa
CK1δ		44 kDa
CK1ε		43 kDa
CLOCK		96 kDa
BMAL1		69 kDa

Molecular weights of the PERCRY–CK1 and CLOCK–BMAL1complexes were calculated (as described in Methods) as the product of 4205 x Stokes radius x sedimentation coefficient. Components of the PER–CRY–CK1 repressor complex are shown; however, it is not known if a single repressor complex exists or if there is a population of repressors, each with different amounts/stoichiometries of the repressor proteins. Below the two complexes, individual clock proteins are listed with their masses, calculated from their sequence. The experimentally derived mass of the CLOCK–BMAL1 complex is consistent with a heterodimer, although somewhat larger than the calculated value. This discrepancy may be partly due to protein modifications present in the complex. The three experiments gave the same values for the complex masses since the peak fraction for the complex was the same in relation to the standards in each experiment. As a way to assess potential variability in the results, we calculated complex molecular weights assuming the peak elution fraction was one fraction earlier or later. In the case of gel filtration, if peak elution had been one fraction earlier or later, then the 707-kDa complex would have been 875 or 589 kDa, respectively, and the 255-kDa complex would have been 336 or 171 kDa. In the case of glycerol gradients, if peak elution had been one fraction earlier or later, then the 707-kDa complex would have been 793 or 557 kDa, respectively, and the 255-kDa complex would have been 282 or 215 kDa.