Cellular and subcellular distribution of saporins, type-1 ribosome-inactivating proteins, in soapwort (Saponaria officinalis L.)

Many plants contain ribosome-inactivating proteins (RIPs) which are either single enzymatically active polypeptides (type-1 RIPs) or heterodimers (type-2 RIPs) composed of an A-chain, functionally equivalent to a type-1 RIP, which is disulphide bonded to a sugar-binding B-chain. Much attention has focused on the use of RIPs as components of immunotoxins or, more recently, as antiviral agents. In contrast, relatively little is known about either the synthesis and targeting of RIPs or their role within plants. In this study the cellular and subcellular distributions of saporins, the type-1 RIPs from soapwort, have been determined in seeds using immunogold labelling. Saporins were present in the seed storage tissue (perisperm), but are not synthesised in the developing embryo, demonstrating that the expression of saporin genes is subject to tissue-specific control. Within the perisperm, saporin was found in extracellular spaces, in the paramural region between the primary wall and plasmalemma and within the vacuole. In addition, saporin was localised in leaf intercellular spaces. This dual localisation, both vacuolar and extracellular, is significantly different from the localisation of ricin, a type-2 RIP found in castor beans, which is targeted to endosperm protein bodies, and to pokeweed antiviral protein which accumulates in the cell wall matrix of leaf mesophyll cells.