Table 1.

Molecular components of taste bud cell types in fungiform and circumvallate papillae.

Taste Cell Type	Taste Detection	Channel/Receptor/ Molecular Marker	Fungiform Papilla	Circumvallate Papilla
Type I	Salt	Epithelial sodium ion channels	65% Amiloride-sensitive [50]	Amiloride-insensitive [50,51]
			Comprised of α, β, and γ subunits [51]	50% cells do not have α, β, and γ subunits [51]
			Rich in β and γ subunits [51,52]	α subunit is predominant [52]
Type II	Sweet (Tas1r2 + Tas1r3) and umami (Tas1r1 + Tas1r3)	GPCR Tas1r1	Predominant expression [53,54,55]	Rare expression [53,54,55]
		GPCR Tas1r2	Rare expression [53,54]	Predominant expression [53,54]
		GPCR Tas1r3	In all of α-gustducin-expressing cells [53,56]	In 10% of α-gustducin-expressing cells [56,57]
	Bitter and toxic	GPCR Tas2rs	Rare expression [42,58]	Predominant expression [42,58,59]
	Sweet, bitter, umami	G protein subunit, Gustducin	α subunit is predominant [31]	α14 subunit is predominant [60,61]
		Galectin-3	In 66% of α-gustducin-expressing cells [42]	In 6% of α-gustducin-expressing cells [42]
		Receptor IP3R3 and PLCβ2	Expressed [62,63]	Expressed in increased proportions [30]
		Transient receptor potential (TRP) channel subfamily TRPM5	A few TRPM5+ cells do not have the Type II cell electrophysiological properties [48]	Always have the Type II cell electrophysiological properties [48]
Type III	Sour	TRP channel member PKD2L1	Expressed, its deletion reduced response to sour stimuli [64,65]	Expressed in increased proportion, deletion has no effect on sour response [64,65]
		TRP channel member PKD1L3	Expression not observed [64,65]	Expressed [64,65]
		Synaptic protein SNAP25	Expressed, a subset lacks PKD2L1 [49]	Expressed in increased proportion, highly coincident with PKD2L1 [49]
		GAD67 (for neurotransmitter GABA synthesis)	Expressed, significant subsets without PKD2L1 or SNAP25 [43,49]	Expressed in increased proportion, coincident with PKD2L1 or SNAP25 [49]
		Synaptic protein Car4	Co-expressed with SNAP25 and GAD67 [66]	A subset lacks SNAP25 and GAD67 [66]
		Neural cell adhesion molecule (NCAM)	Car4-NCAM+ in Type III cells, also co-labels with Type II marker IP3R3 [67]	Expressed only in Type III cells [67]