TABLE 3.

Cellular levels of Upf1p, Nmd2p, and Upf3p^a

Protein or ribosomes	Protein
	No. of molecules per cell	% Soluble
Upf1p	1,600	0.007
Nmd2p	160	0.0007
Upf3p	80	0.00035
Sup45p	29,600b	0.2b
Xrn1p	29,000c	0.2c
Ribosomes	300,000d	7

^aThe relative abundance of Upf1p and Nmd2p was determined by comparing Western blot band intensities of the UPF/NMD factors present in crude cell extracts to those of the individual proteins. For Upf1p and Nmd2p, highly purified recombinant GST-Upf1p and GST-Nmd2p were used as standards, respectively. For Upf3p, cells bearing an HA-NMD2 allele were used as the standard. Calculations used to derive protein abundance are summarized in Materials and Methods. 

^bCellular Sup45p levels determined by Gygi et al. (16). 

^cCellular Xrn1p levels determined by Heyer et al. (25). 

^dCellular levels of ribosomes determined by Waldron and Lacroute (56).