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.