yeast strain. Transformants were assayed by genomic PCR with one
primer specific for the GFP tag and a second specific for each ORF,
to determine whether the cassette had integrated at the appropriate
locus. A total of 6,029 strains with chromosomally GFP-tagged
ORFs were grown to mid-logarithmic phase in synthetic medium
and analysed by fluorescence microscopy; 4,156 of these showed
GFP signals above background levels (Table 1).
Micrographs of each GFP-tagged strain (Fig. 1b; see also Supplementary
Fig. S1), lacking ORF identifiers, were independently
evaluated by two scorers and initially classified into one or more of
12 subcellular localization categories (Table 2). We then refined
these categories by performing a series of co-localization experiments.
Haploid reference strains expressing monomeric red fluorescent
protein (mRFP)20 fusions to proteins whose localization had
been characterized previously (Table 2) were mated to approximately
700 GFP strains that were not assigned definitive localizations
by GFP microscopy alone, and the resulting diploid cells were
analysed by fluorescence microscopy (Fig. 1c). On the basis of this
analysis, proteins were assigned to an additional 11 localization
categories (Table 2). All information was captured into a database
(http://yeastgfp.ucsf.edu).