MVC validation on JQuery UI Autocomplete (invalid values)

Question

I'm trying to validate an autocomplete field using ASP.NET MVC and I have a doubt. I'll describe the scenario below:

Currently, i'm retrieving the values from an Ajax call in the Form controller, like this:

$("#AutoComplete").autocomplete({
            source: function (request, response) {
                $.ajax({
                    url: "/form/AjaxCall",
                    type: "POST",
                    dataType: "json",
                    data: { term: request.term },
                    success: function (data) {
                        response($.map(data, function (item) {
                            return { label: item.nome, value: item.nome };
                        }))

                    }
                })
            },
        });

My Form controller:

    public ActionResult AjaxCall(string term)
    {
        var result = (from r in db.Elementoes
                      where r.nome.ToLower().Contains(term.ToLower())
                      select new { r.nome }).Distinct();
        return Json(result, JsonRequestBehavior.AllowGet);
    }

My View field:

@Html.TextBoxAutoCompleteFor(model => model.AutoComplete)

Let's suppose that the available values on the autocomplete are: Acre, Piracicaba and Jundiaí

How can I validate the user's input to the field (client and server-side) and show a validation message like: "the text wasn't found on the database?" Example: If he types Acre it's ok, but if he types Acre3 the error is showed while typing?

Some points to consider:

1 - I cannot set the autocomplete value to empty if the value isn't correct, i need to show the message.

2 - I cannot set the error class manually in jQuery. I need to use jQuery unobtrusive validation.

I referred to the following articles, but those solutions doesn't worked:

MVC Autocomplete validation

MVC validation with autocomplete field

Thanks in advance!