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Similar to multicellular organisms, single-celled eukaryotes like yeast communicate using cell surface receptor proteins.

For example, during sexual reproduction, yeast form two haploid cell types called a and α mating types, each with a single set of chromosomes.

Each type secretes specific mating factors that can bind to and activate the G-protein coupled receptors or GPCRs on the cells of the opposite mating type.

The activated GPCR undergoes a conformational change and binds the G-protein, exposing its nucleotide-binding site to trigger GDP exchange for GTP.

Next, the G-protein dissociates from the receptor and its subunits separate into two groups.

Both these groups of G-protein subunits can individually bind and activate effectors which use secondary messengers such as cyclic adenosine monophosphate to ultimately upregulate the transcription of mating-specific genes.

Additionally, these signals cause the yeast cell to grow toward the nearby mating partner, a process known as shmooing.

First the membranes of the haploid cells merge, followed by fusion of their nuclei, resulting in a diploid yeast cell.