Different properties of two types of auxin-binding sites in membranes from maize coleoptiles

Two types of auxin-binding sites (sites I and II) in membranes from maize (Zea mays L.) coleoptiles were characterized. Site I was a protein with a relative molecular mass of 21 000, and the distribution of site I protein on sucrose density gradient fractionation coincided with that of NADH-cytochrome-c reductase (EC 1.6.99.3), a marker enzyme of the endoplasmic reticulum. Immunoprecipitation and immunoblotting studies showed that the content of site I protein in maize coleoptiles was approx. 2 μg·(g FW)(-1). Site II occurred in higher-density fractions and also differed immunologically from site I. Site I was present at the early developmental stage of the coleoptile and increased only twice during coleoptile growth between day 2 and 4. Site II activity was low at the early stage and increased more substantially between day 3 and 4, a period of rapid growth of the coleoptile. Both sites decreased concurrently after day 4, followed by a reduction in the growth rate of the coleoptile. Coleoptiles with the outer epidermis removed showed a lower site I activity than intact coleoptiles, indicating that site I was concentrated in the outer epidermis. Site II, in contrast, remained constant after removal of the outer epidermis. The results indicate that site I is not a precursor of site II and that the two sites are involved in different cellular functions.