Self-association of the adenoviral L4-22K protein

Human adenovirus (Ad) is an icosahedral, double-stranded DNA virus that causes infections of the respiratory tract, urinary tract, and gastrointestinal tract. Assembly of virus particles requires condensation and encapsidation of the linear viral genome. This process requires sequence specific binding of two viral proteins, called IVa2 and L4-22K, to a conserved sequence located at the left end of the viral genome, called the packaging sequence (PS). IVa2 and an alternatively spliced form of L4-22K, called L4-33K, also function as transcriptional activators of the major late promoter (MLP), which encodes viral structural and core proteins. IVa2 and L4-33K bind to identical conserved DNA sequences downstream of the MLP, called the downstream element (DE), to activate transcription. To begin to dissect how the IVa2, L4-22K, and L4-33K proteins simultaneously function as transcriptional activators and DNA packaging proteins, we need to understand the thermodynamics of assembly of these proteins on DNA that contains the PS as well as the DE. Toward this end, we have characterized the self-assembly properties of highly purified, recombinant L4-22K protein. We show that L4-22K reversibly assembles into higher-order structures according to an indefinite, isodesmic assembly scheme. We show that the smallest polymerizing unit is likely the L4-22K monomer (s(20,w) = 2.16 ± 0.04 S) and that the monomer assembles with itself and/or other aggregates with an equilibrium association constant, L, of 112 (102, 124) μM(-1) (0.1 M NaCl, pH 7, 25 °C). A mechanistic consequence of an isodesmic, indefinite assembly process is that the free concentration of the smallest polymerizing unit cannot exceed 1/L. We discuss the implications of this observation with respect to the thermodynamics of assembly of L4-22K and IVa2 on the PS.