Thanks krew, but I must honestly say, maybe the title should say "test mule" instead of "prototype". Nothing wrong with calling it prototype, as there are various stages of prototyping and this stage is very much one of them. It's just much easier if people can distinguish a "test mule"---that utilizes the incumbent generation's body shell---against a running test vehicle of the production design and packaged components in the form of a "prototype".
I can see some people casually expressing premature disappointment/puzzlement, even after all your efforts to clarify in the body of text below, that this is just a mule using the current LS body. This site is very good and highly accurate, so one is rarely confused when reading anything here. Actual 5LS prototypes have been running since last year, but not yet outside of high-security Lexus test facilities in Japan surely. I think this might be testing an alternative powertrain that may launch in 2017, some months after the petrol variants.
The final exterior design had to have been reached by late 2013, if not fully finalised then. 36 months from then to production is usually the time frame on such a large product and pretty much a typical pattern for the LS.
Akio Toyoda, as per the words of a well-regarded Lexus salesman and enthusiast, is "personally invested in this project" more than anything. It will be world-class apparently, but one hopes it holds true. This is the car that he is constantly chaffeured in (current LS600hL), so I am sure like Dr. Dieter Zetsche of Daimler AG did with the W222 S-Class, he wants something to his taste that he can enjoy day-in, day-out.
The LX being introduced in Japan this year, might also have to do with his own interests to a degree and wanting more than a Land Cruiser as an alternative. Mary Barra of GM is chauffeured in an Escalade daily, so it is not too shocking how that happened.
In my own work at my job, I have stressed this philosophy to any complacent superiors, on why our best offering must receive the highest possible level of investment, while not being a loss maker. Image building, conquest customers, absolute quality, and innovation.