There are no known functional or structural relationships between tubulins and P-proteins, even though both groups of proteins share the general characteristic of appearing as linear macromolecular assemblies. After actin, tubulin is possibly the second most commonly occurring protein of the non-specialized eukaryote cell; P-protein on the other hand is the major intracellular polymeric component of a highly specialized plant cell, the sieve tube. Each system offers a model situation, regulated temporally and spatially, for studying the assembly of a relatively few distinct components into highly ordered cellular structures.