ULTRASTRUCTURAL EVIDENCE FOR A BINDING SUBSTANCE TO THE SWEET-TASTINGPROTEIN THAUMATIN INSIDE TASTE BUD PORES OF RHESUS-MONKEY FOLIATE PAPILLAE

Thaumatin is a protein that tastes intensely sweet only to Old World m onkeys and to higher primates, including man. Here we used pre-embeddi ng ultrastructural methods to study the distribution of thaumatin in a pical regions of Rhesus monkey foliate papillae, using thaumatin conju gated to 5 nm gold particles. With freeze-substitution we saw that gol d-labeled thaumatin bound to an electron-opaque, sponge-like secretory substance inside the taste bud pores. Labeled thaumatin was found at the surface of the secretory substance even deep inside the pore, wher e other, unlabeled cellular structures surrounded the substance. With freeze-fracture deep-etching the secretory substance that bound the th aumatin-gold particles appeared coarsely granular. There was no labeli ng of any other taste bud pore structure, including microvilli and sma ll membrane-lined vesicles. Pre-incubation with an excess of unlabeled thaumatin inhibited binding with gold-labeled thaumatin. The results suggest that the secretory substance had the greatest affinity of all taste pore structures to the sweet-tasting compound under our experime ntal conditions. Therefore, gustatory reception probably involves vari ous taste compound binding structures, microvilli, and also secretory substances like the one described here which bound thaumatin. We specu late that the secretory substance may bind taste stimuli and serve as an intermediate between stimuli and receptors. It could be involved in stimulus removal or delivery or both. (C) 1993 Wiley-Liss, Inc.